docs: add RELEASE.md

/cc https://github.com/heroku/terraform-provider-heroku/blob/master/RELEASE.md

RELEASE.md

@@ -0,0 +1,30 @@
+## Release Flow
+
+Since the migration to the [Terraform registry](https://registry.terraform.io/), this repository's maintainers now have
+the ability to self-publish Terraform GitHub provider releases. This process leverages Github Actions
+and [`goreleaser`](https://github.com/goreleaser/goreleaser) to build, sign, and upload provider binaries to a Github release.
+
+The release flow is as follows:
+1. Create a CHANGELOG entry for the release using the following format:
+    ```markdown
+    ## x.y.z (release date)
+
+    ## ENHANCEMENTS:
+    ...
+
+    ## BUG FIXES:
+    ...
+    ```
+1. Tag the commit that adds the CHANGELOG entry with the release version and push:
+    ```shell
+    $ git tag x.y.z
+    $ git push origin x.y.z
+    ```
+1. Github Actions will trigger the release workflow which can be
+[viewed here](https://github.com/integrations/terraform-provider-github/actions?query=workflow%3Arelease).
+After the workflow executes successfully, the Github release created in the prior step will
+have the relevant assets available for consumption.
+1. The new release will show up in https://registry.terraform.io/providers/integrations/github/latest for consumption
+by terraform `0.13.X` users.
+1. For terraform `0.12.X` users, the new release is available for consumption once it is present in
+https://releases.hashicorp.com/terraform-provider-github/.