Add a bit more to the readme (#625)
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.
GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples