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.

.env.sample

@@ -15,6 +15,9 @@ COMPOSE_FILE="compose.yml"
 
 #DISABLE_FEDERATION=1
 
+# Set "true" to enable federation endpoint on $DOMAIN/.well-known/matrix/server
+SERVE_SERVER_WELLKNOWN=false 
+
 ENABLE_REGISTRATION=false
 PASSWORD_LOGIN_ENABLED=true