Update nextcloud.md

updated nextcloud service docs with redis support info

docs/services/nextcloud.md

@@ -9,6 +9,7 @@ This service requires the following other services:
 
 - a [Postgres](postgres.md) database
 - a [Traefik](traefik.md) reverse-proxy server
+- a [Redis](redis.md) data-store (optional)
 
 
 ## Configuration
@@ -27,6 +28,9 @@ nextcloud_enabled: true
 nextcloud_hostname: mash.example.com
 nextcloud_path_prefix: /nextcloud
 
+# Enable redis (optional)
+#nextcloud_redis_enabled: true
+
 ########################################################################
 #                                                                      #
 # /nextcloud                                                           #