docs: Add aliases documentation

The processing of aliases wasn't properly documented in an user-visible
way, so this patch adds a new section for it.

docs/aliases.md

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+
+# Aliases
+
+[chasquid] supports [email aliases], which is a mechanism to redirect mail
+from one account to others.
+
+
+## File format
+
+The aliases are configured per-domain, on a text file named `aliases` within
+the domain directory. So like `/etc/chasquid/domains/example.com/aliases`.
+
+The format is very similar to the one used by classic MTAs (sendmail, exim,
+postfix), but not identical.
+
+### Comments
+
+Lines beginning with `#` are considered comments, and are ignored.
+
+### Email aliases
+
+To create email aliases, where mail to a user are redirected to other
+addresses, write lines of the form `user: address, address, ...`.
+
+The user should not have the domain specified, as it is implicit by the
+location of the file. The domain in target addresses is optional, and defaults
+to the user domain if not present.
+
+For example:
+
+```
+# Redirect mail to pepe@ to jose@ on the same domain.
+pepe: jose
+
+# Redirect mail to flowers@ to the indvidual flowers.
+flowers: rose@backgarden, lilly@pond
+```
+
+User names cannot contain spaces, ":" or commas, for parsing reasons. This is
+a tradeoff between flexibility and keeping the file format easy to edit for
+people. User names will be normalized internally to lower-case. UTF-8 is
+allowed and fully supported.
+
+### Pipe aliases
+
+A pipe alias is of the form `user: | command`, and causes mail to be sent as
+standard input to the given command.
+
+The command can have space-separated arguments (no quotes or escaping
+expansion will be performed).
+
+For example:
+
+```
+# Mail to user@ will be piped to this command for delivery.
+user: | /usr/bin/email-handler --work
+
+# Mail to null@ will be piped to "cat", effectively discarding the email.
+null: | cat
+```
+
+
+## Processing
+
+Aliases files are read upon start-up and refreshed every 30 seconds, so
+changes to them don't require a daemon restart.
+
+The resolver will perform lookups recursively, until it finds all the final
+recipients. There are recursion limits to avoid alias loops. If the limit (10
+levels) is reached, the entire resolution will fail.
+
+Commands are given 30s to run, after which it will be killed and the execution
+will fail.  If the command exits with an error (non-0 exit code), the delivery
+will be considered failed.
+
+The `chasquid-util` command-line tool can be used to check and resolve
+aliases.
+
+
+[chasquid]: https://blitiri.com.ar/p/chasquid
+[email aliases]: https://en.wikipedia.org/wiki/Email_alias