docs(login): Use client and add note for cloud. (#56)

Author: danielmai

README.md

@@ -106,13 +106,14 @@ which will allow the users to login to a specific namespace.
 In order to create a JavaScript client, and make the client login into namespace `123`:
 
 ```js
-const dgraphClientStub = new dgraph.DgraphClientStub("localhost:9080");
-await dgraphClientStub.loginIntoNamespace("groot", "password", 123); // where 123 is the namespaceId 
+await dgraphClient.loginIntoNamespace("groot", "password", 123); // where 123 is the namespaceId 
 ```
 
 In the example above, the client logs into namespace `123` using username `groot` and password `password`.
 Once logged in, the client can perform all the operations allowed to the `groot` user of namespace `123`.
 
+If you're connecting to Dgraph Cloud, call `setCloudApiKey` before calling `loginIntoNamespace`.
+
 ### Create a Client for Dgraph Cloud Endpoint
 
 If you want to connect to Dgraph running on your [Dgraph Cloud](https://cloud.dgraph.io) instance, then all you need is the URL of your Dgraph Cloud endpoint and the API key. You can get a client using them as follows: