Write docs on steps necessary for supporting a new service.

[ajkannan]

This readme should contain items such as (but not limited to):

• Splitting code into distinct SPI and API layer
• Writing local test helpers if an emulator doesn't already exist (and discuss standards for a test helper/emulator)
• Documentation (package info, step-by-step guide, other docs that need to be updated)
• Adding example code to gcloud-java-examples
• Making sure any versioned jars/docs/etc are included in the update_pom_version.sh script
• Add the example to the list of examples in gcloud-java-examples/pom.xml in the assembler plugin
• Look at the SLA to determine the appropriate default retry settings (see When using default retry settings we should get that default from the specific service options #857)
• Be aware that library version in READMEs is auto-updated by utilities/update_docs_version.sh when releasing.
• Mention that code should be added to gcloud-java-examples/test-apps for each new service.

This doc can be linked from the CONTRIBUTING docs page.