updated mongo version and replicaSet requirement

rocket.chat/content.md

@@ -8,16 +8,20 @@ It is a great solution for communities and companies wanting to privately host t
 
 # How to use this image
 
-First, start an instance of mongo:
+First, start an instance of mongo and initiate replicaSet:
 
 ```console
-$ docker run --name db -d mongo:3.0 --smallfiles
+$ docker run --name db -d mongo:4.0 --smallfiles --replSet rs0 --oplogSize 128
+```
+
+```console
+$ docker exec -ti db mongo --eval "printjson(rs.initiate())"
 ```
 
 Then start Rocket.Chat linked to this mongo instance:
 
 ```console
-$ docker run --name rocketchat --link db -d %%IMAGE%%
+$ docker run --name rocketchat --link db --env MONGO_OPLOG_URL=mongodb://db:27017/local -d %%IMAGE%%
 ```
 
 This will start a Rocket.Chat instance listening on the default Meteor port of 3000 on the container.
@@ -25,7 +29,7 @@ This will start a Rocket.Chat instance listening on the default Meteor port of 3
 If you'd like to be able to access the instance directly at standard port on the host machine:
 
 ```console
-$ docker run --name rocketchat -p 80:3000 --env ROOT_URL=http://localhost --link db -d %%IMAGE%%
+$ docker run --name rocketchat -p 80:3000 --link db --env ROOT_URL=http://localhost --env MONGO_OPLOG_URL=mongodb://db:27017/local -d %%IMAGE%%
 ```
 
 Then, access it via `http://localhost` in a browser. Replace `localhost` in `ROOT_URL` with your own domain name if you are hosting at your own domain.
@@ -33,5 +37,5 @@ Then, access it via `http://localhost` in a browser. Replace `localhost` in `ROO
 If you're using a third party Mongo provider, or working with Kubernetes, you need to override the `MONGO_URL` environment variable:
 
 ```console
-$ docker run --name rocketchat -p 80:3000 --env ROOT_URL=http://localhost --env MONGO_URL=mongodb://mymongourl/mydb -d %%IMAGE%%
+$ docker run --name rocketchat -p 80:3000 --env ROOT_URL=http://localhost --env MONGO_URL=mongodb://mymongourl/mydb --env MONGO_OPLOG_URL=mongodb://mymongourl:27017/local -d %%IMAGE%%
 ```