Create README files for all APIs

[jabubake]

Few that I found :

• https://github.com/GoogleCloudPlatform/google-cloud-java/tree/master/google-cloud-dep-verification
• https://github.com/GoogleCloudPlatform/google-cloud-java/tree/master/google-cloud-trace