Update README.md

README.md

@@ -158,7 +158,7 @@ Environment Variables
     * NODE_ENV: `production` OR `development`.
     * PORT: Server port. default: `3000`
     * ELASTICSEARCH_URI: URI to connect to Elasticearch.
-    * REDIS_URI: URI to connect to Redis (to session store).
+    * REDIS_URI: URI to connect to Redis (use it as a session store instead of MongoDB).
     * PLANTUML_URI: URI to connect to [PlantUML](http://plantuml.com/) server.
     * BLOCKDIAG_URI: URI to connect to [blockdiag](http://http://blockdiag.com/) server.
     * PASSWORD_SEED: A password seed used by password hash generator.