add SERVE_SERVER_WELLKNOWN variable

This config allows matrix-synapse to create a endpoint on
$DOMAIN/.well-known/matrix/server used for federation.
It's a straight forward way (no port opening required)
to enable federation if you are okay having a subdomain
(matrix.example.org) as your homeserver address.

homeserver.yaml.tmpl

@@ -85,7 +85,7 @@ public_baseurl: https://{{ env "DOMAIN" }}/
 #
 # Defaults to 'false'.
 #
-#serve_server_wellknown: true
+serve_server_wellknown: {{ env "SERVE_SERVER_WELLKNOWN" }}
 
 # Set the soft limit on the number of file descriptors synapse can use
 # Zero is used to indicate synapse should set the soft limit to the