docs(readme): improve client.js usage docs with JWT and API examples

Author: hivanalejandro

endpoints/getting-started-grpc/README.md

@@ -13,10 +13,29 @@ $ node server.js -p 50051
 ```
 
 ### Run the client
+For running the client locally, you'll need either an API key or a JWT auth token:
+
+#### Using a JWT token (recommended for local development)
+You can generate a development-only JWT token using Google Cloud CLI:
+
+```
+$ gcloud auth print-identity-token
+```
+See [print-identity-token](https://cloud.google.com/sdk/gcloud/reference/auth/print-identity-token) for more info.
+
+Then run the client using this token:
 ```
-$ node client.js -h localhost:50051
+$ node client.js -h localhost:50051 -j YOUR_JWT_TOKEN
 ```
 
+#### Using an API key
+Alternatively, you can use an API key from your Google Cloud project:
+```
+$ node client.js -h localhost:50051 -k YOUR_API_KEY
+```
+
+You can create API keys in the [Google Cloud Console](https://console.cloud.google.com/apis/credentials).
+
 ## Running on Google Cloud Platform
 ### Setup
 Make sure you have [gcloud][gcloud] and [Node.js][nodejs] installed.