Add a bit more to the readme (#625)

1. Put an index up (Note that we have to GAE Standard samples at the top level)
2. Add some info about credentials.

README.md

@@ -6,3 +6,41 @@
 
 This is a repository that contains java code snippets on [Cloud Platform Documentation](https://cloud.google.com/docs/).
 
+The repo is organized as follows:
+
+* [App Engine Standard](appengine)
+  * [TaskQueue](taskqueue) <!-- shouldn't this be in appengien ?? -->
+  * [Unit Tests](unittests)
+* [App Engine Flexible](flexible)
+* [Compute Engine](compute)
+
+Technology Samples:
+
+* [Bigquery](bigquery)
+* [Datastore](datastore)
+* [Endpoints](endpoints)
+* [Key Management Service](kms)
+* [Logging](logging)
+* [Monitoring](monitoring)
+* [Natural Language](language)
+* [PubSub](pubsub)
+* [Cloud Spanner](spanner)
+* [Speech](speech)
+* [Cloud Storage](storage)
+* [Translate](translate)
+* [Vision](vision)
+
+## Credentials Example
+
+The documentation for [Application Default Credentials](https://developers.google.com/identity/protocols/application-default-credentials).
+
+`BigQuery bigquery = BigQueryOptions.getDefaultInstance().getService();`
+
+The client library looks for credentials using the following rules:
+
+1. `GOOGLE_APPLICATION_CREDENTIALS` environment variable, pointing to a service account key JSON file path.
+2. Cloud SDK credentials `gcloud auth application-default login`
+3. App Engine standard environment credentials.
+4. Compute Engine credentials.
+
+You can override this behavior using setCredentials in `BigQueryOptions.newBuilder()` by adding `setCredentials(Credentials credentials)` from [ServiceOptions.builder](http://googlecloudplatform.github.io/google-cloud-java/0.12.0/apidocs/com/google/cloud/ServiceOptions.Builder.html#setCredentials-com.google.auth.Credentials-) and [Credentials](http://google.github.io/google-auth-library-java/releases/0.6.0/apidocs/com/google/auth/Credentials.html?is-external=true).