Add back build instructions, contribution workflow, and checkstyle tools to contributing guide (#1256)
Add back build instructions, contribution workflow, and checkstyle tools to contributing guide (#1256)

Updates to CONTRIBUTING.md:

- Adds back build instructions from the old repo’s contribution guide, along with (slightly adapted) steps for a typical contribution workflow
- Adds links to google-java-format tool and plugins
GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp