Add documentation for multi-remote setups.

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+% Multi-remote setup for InterIMAP
+% [Guilhem Moulin](mailto:guilhem@fripost.org)
+
+This document describes the setup of InterIMAP against two remotes
+accounts, a *personal* account at `imap.example.net`, and a *work*
+account at `imap.example.com`.  (The same setup can be adapted to remote
+accounts or more.)  These remotes accounts are to be synchronized
+locally, each being mirrored to its own local namespace in a
+bidirectional fashion.
+
+We assume familiarity with the [Getting Started] tutorial.
+Prerequisites, such as installing dependencies, creating directories,
+etc., won't be repeated here.
+
+
+Layout
+======
+
+For this example we assume the *personal* account has the following
+mailbox hierarchy (with delimiter `/`):
+
+    INBOX
+    debian
+    debian/debian-devel
+    debian/debian-project
+    debian/debian-security
+    archives
+
+And the *work* account the following mailbox hierarchy (with delimiter `.`):
+
+    INBOX
+    INBOX.todo
+    attic
+
+These remote mailbox hierarchies are to be mirrored locally as:
+
+    INBOX
+    perso/INBOX
+    perso/debian
+    perso/debian/debian-devel
+    perso/debian/debian-project
+    perso/debian/debian-security
+    perso/archives
+    work/INBOX
+    work/INBOX/todo
+    work/attic
+
+Using again (arbitrarily) `/` as local hierarchy delimiter.  (Since 0.5
+each server can choose its own hierarchy delimiter independently on the
+other ones, and even change it — for instance after changing the server
+software — without breaking synchronization.)
+
+Note that there are alternative layouts (one can for instance replace
+the prefix `perso/` with the empty string), however for this example
+we'll focus on the above layout.
+
+
+Local Dovecot configuration
+===========================
+
+Although it's not required (it's possible to use the default namespace
+for everything), we propose to use a dedicated [IMAP namespace][RFC
+2342] for each remote account to mirror locally:
+
+ * It provides better isolation of the locally mirrored accounts: one
+   can use entirely independent mail storage for instance, and even
+   different hierarchy delimiters.
+ * One can use `perso/INBOX` as the local `INBOX`.  (Per [RFC 3501] the
+   `INBOX` mailbox has a special nature.)  Or even alias a particular
+   namespace to the default namespace.  That being said, one should use
+   aliases with care as they might have undesired side effects, such as
+   `perso/INBOX` missing from `LIST` responses when the mailbox is used
+   as the local `INBOX`; or confusion between `perso/work/foo` and
+   `work/foo` when `perso/` is aliased to the default namespace.
+
+We define 3 namespaces: a default namespace holding the local `INBOX`,
+as well as a dedicated namespace — with a suitable prefix — for each
+remote account to mirror locally.  Consult the [Dovecot namespaces]
+documentation for more information.  Note that mailboxes (and the
+messages they contain) residing in the default namespace won't be copied
+over: they will only live on the local instance.
+
+    $ cat >${XDG_CONFIG_HOME:-~/.config}/dovecot/dovecot.conf <<-EOF
+		ssl = no
+		namespace {
+		    location  = maildir:~/Mail
+		    inbox     = yes
+		    separator = /
+		}
+		namespace perso {
+		    prefix    = perso/
+		    location  = maildir:~/Mail/perso
+		    separator = /
+		}
+		namespace work {
+		    prefix    = work/
+		    location  = maildir:~/Mail/work
+		    separator = /
+		}
+	EOF
+
+
+We can see the three namespaces using the `` `~/.local/bin/dovecot-imap` ``
+wrapper defined in the [Getting Started] tutorial.
+
+    $ ~/.local/bin/dovecot-imap
+    S: * PREAUTH [CAPABILITY IMAP4rev1 …] Logged in as myuser
+    C: a NAMESPACE
+    S: * NAMESPACE (("" "/")("work/" "/")("perso/" "/")) NIL NIL
+    S: a OK Namespace completed (0.001 + 0.000 secs).
+    C: b LIST "" "*"
+    S: * LIST (\Noselect \HasNoChildren) "/" work
+    S: * LIST (\Noselect \HasNoChildren) "/" perso
+    S: * LIST (\HasNoChildren) "/" INBOX
+    S: b OK List completed (0.001 + 0.000 secs).
+    C: q LOGOUT
+    S: * BYE Logging out
+    S: q OK Logout completed (0.001 + 0.000 secs).
+
+
+InterIMAP configuration
+=======================
+
+We use a dedicated [`interimap`(1)] instance for each remote account to
+synchronize, starting with the *personal* account.  Again, see the
+[Getting Started] guide for details about the configuration file and its
+settings.
+
+    $ install -m0600 /dev/null ${XDG_CONFIG_HOME:-~/.config}/interimap/personal
+<!-- -->
+    $ cat >${XDG_CONFIG_HOME:-~/.config}/interimap/personal <<-EOF
+		database = personal.db
+
+		[local]
+		type = tunnel
+		list-reference = perso/
+		command = exec ~/.local/bin/dovecot-imap
+
+		[remote]
+		type = imaps
+		host = imap.example.net
+		username = myname
+		password = xxxxxxxx
+	EOF
+<!-- -->
+    $ interimap --config=personal
+    Creating new schema in database file …/personal.db
+    database: Created mailbox INBOX
+    local: Created mailbox perso/INBOX
+    […]
+
+And similarly for the *work* account:
+
+    $ install -m0600 /dev/null ${XDG_CONFIG_HOME:-~/.config}/interimap/work
+<!-- -->
+    $ cat >${XDG_CONFIG_HOME:-~/.config}/interimap/work <<-EOF
+		database = work.db
+
+		[local]
+		type = tunnel
+		list-reference = work/
+		command = exec ~/.local/bin/dovecot-imap
+
+		[remote]
+		type = imaps
+		host = imap.example.com
+		username = myname2
+		password = xxxxxxxx
+	EOF
+<!-- -->
+    $ interimap --config=work
+    Creating new schema in database file …/work.db
+    database: Created mailbox INBOX
+    local: Created mailbox work/INBOX
+    […]
+
+The local mail storage should now have the desired local layout:
+
+    $ ~/.local/bin/dovecot-imap
+    S: * PREAUTH [CAPABILITY IMAP4rev1 …] Logged in as myuser
+    C: b LIST "" "*"
+    S: * LIST (\Noselect \HasChildren) "/" work
+    S: * LIST (\HasNoChildren) "/" work/attic
+    S: * LIST (\HasChildren) "/" work/INBOX
+    S: * LIST (\HasChildren) "/" work/INBOX
+    S: * LIST (\HasNoChildren) "/" work/INBOX/todo
+    S: * LIST (\Noselect \HasChildren) "/" perso
+    S: * LIST (\HasNoChildren) "/" perso/INBOX
+    S: * LIST (\HasChildren) "/" perso/debian
+    S: * LIST (\HasNoChildren) "/" perso/debian/debian-security
+    S: * LIST (\HasNoChildren) "/" perso/debian/debian-project
+    S: * LIST (\HasNoChildren) "/" perso/debian/debian-devel
+    S: * LIST (\HasNoChildren) "/" perso/archives
+    S: * LIST (\HasNoChildren) "/" INBOX
+    S: a OK List completed (0.003 + 0.000 + 0.002 secs).
+    C: q LOGOUT
+    S: * BYE Logging out
+    S: q OK Logout completed (0.001 + 0.000 secs).
+
+Template user unit for systemd are provided in order to run these
+[`interimap`(1)] instances as services:
+
+    $ systemctl --user enable --now interimap@{personal,work}.service
+
+[Getting Started]: getting-started.html
+[RFC 2342]: https://tools.ietf.org/html/rfc2342
+[RFC 3501]: https://tools.ietf.org/html/rfc3501
+[Dovecot namespaces]: https://doc.dovecot.org/configuration_manual/namespace/
+[`interimap`(1)]: interimap.1.html