doc: server metrics require the memcache module

server/README.md

@@ -9,6 +9,7 @@ The game server currently runs on nodejs v0.4.7 (but should run fine on the late
 - websocket
 - websocket-server
 - sanitizer
+- memcache (only if you want metrics)
 
 All of them can be installed via `npm install [module_name]`
 
@@ -38,4 +39,4 @@ The server has a status URL which can be used as a health check or simply as a w
 
 Send a GET request to: `http://[host]:[port]/status`
 
-It will return a JSON array containing the number of players in all instanced worlds on this game server.
+It will return a JSON array containing the number of players in all instanced worlds on this game server.