new: [mattermost] added example, commented.

compose.yml

@@ -85,4 +85,24 @@ letsencrypt:
       application:
         key: XXX
         secret: YYY
-      consumer_key: ZZZ
+      consumer_key: ZZZ
+# mattermost:
+#   charm: mattermost
+#   relations:
+#     web-proxy:
+#       frontend:
+#         domain: mattermost.local             ## <-- modify this
+#         ssl: letsencrypt
+#         apache-custom-rules: |
+#           ProxyPreserveHost On
+#
+#           # Set web sockets
+#           RewriteEngine On
+#           RewriteCond %{REQUEST_URI} /api/v[0-9]+/(users/)?websocket [NC,OR]
+#           RewriteCond %{HTTP:UPGRADE} ^WebSocket$ [NC,OR]
+#           RewriteCond %{HTTP:CONNECTION} ^Upgrade$ [NC]
+#           RewriteRule .* ws://mattermost:8000%{REQUEST_URI} [P,QSA,L]
+#     postgres-database:
+#       postgres:
+#         user: mattermost
+#         dbname: mattermost