Update CONTRIBUTING.md (#798)

Clarify use of release branch

Author: themantissa

CONTRIBUTING.md

@@ -17,5 +17,5 @@ After the above 3 steps are completed and we've agreed on a path forward:
 1. Implement your feature, improvement or bug fix, ensuring it adheres to the [Terraform Plugin Best Practices](https://www.terraform.io/docs/extend/best-practices/index.html)
 1. Ensure you follow the [Terraform Plugin Testing requirements](https://www.terraform.io/docs/extend/testing/index.html).
 1. Add comments around your new code that explain what's happening
-1. Commit and push your changes to your branch then submit a pull request.
+1. Commit and push your changes to your branch then submit a pull request against the current release branch, not master.  The naming scheme of the branch is `release-staging-v#.#.#`. Note: There will only be one release branch at a time.  
 1. A repo maintainer will review the your pull request, and may either request additional changes or merge the pull request.