docs: Document the use of SRS when doing alias forwarding

When an alias has a remote destination, chasquid uses sender rewriting
(also known as SRS [1]) to forward the email without risking being in
violation of SPF policies.

See https://en.wikipedia.org/wiki/Sender_Rewriting_Scheme for more
details.

This, however, wasn't documented anywhere, as noted in
https://github.com/albertito/chasquid/issues/6.

This patch adds a paragraph to the alias documentation explaining this
behaviour.

docs/aliases.md

@@ -36,6 +36,12 @@ pepe: jose
 flowers: rose@backgarden, lilly@pond
 ```
 
+Destination addresses can be for a remote domain as well. In that case, the
+email will be forwarded using
+[sender rewriting](https://en.wikipedia.org/wiki/Sender_Rewriting_Scheme).
+While the content of the message will not be changed, the envelope sender will
+be the constructed from the alias user.
+
 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