Document on how to use Promises (#1357)

Author: sampajano

README.md

@@ -269,6 +269,8 @@ code.
 * `echo_pb.d.ts` - Generated by `--grpc-web_out`, contains TypeScript
 definitions for `echo_pb.js`.
 
+### Using Callbacks
+
 ```ts
 import * as grpcWeb from 'grpc-web';
 import {EchoServiceClient} from './EchoServiceClientPb';
@@ -288,6 +290,21 @@ call.on('status', (status: grpcWeb.Status) => {
 });
 ```
 
+(See [here](https://github.com/grpc/grpc-web/blob/4d7dc44c2df522376394d3e3315b7ab0e010b0c5/packages/grpc-web/index.d.ts#L29-L39) full list of possible `.on(...)` callbacks)
+
+### (Option) Using Promises (Limited features)
+
+NOTE: It is not possible to access the `.on(...)` callbacks (e.g. for `metadata` and `status`) when Promise is used.
+
+```ts
+this.echoService.echo(request, {'custom-header-1': 'value1'})
+  .then((response: EchoResponse) => {
+    console.log(`Received response: ${response.getMessage()}`);
+  }).catch((err: grpcWeb.RpcError) => {
+    console.log(`Received error: ${err.code}, ${err.message}`);
+  });
+```
+
 For the full TypeScript example, see
 [ts-example/client.ts](net/grpc/gateway/examples/echo/ts-example/client.ts) with the [instructions](net/grpc/gateway/examples/echo/ts-example) to run.