Add release instructions. (#128)

Author: dlorenc

RELEASING.md

@@ -0,0 +1,56 @@
+# Releasing container-diff
+
+The github.com/GoogleCloudPlatform/container-diff uses Container Builder triggers to build and release binaries.
+These triggers are setup via the Cloud Console, but the builds they execute live in this repo.
+
+## Continuous Builds
+
+Every commit to master is built and pushed automatically to a GCS location named via the COMMIT_SHA.
+
+```shell
+$ gsutil ls gs://container-diff/builds/
+gs://container-diff/builds/b726215b8b978e1d85257af5c2fd0a6fb8e116fd/
+
+$ gsutil ls gs://container-diff/builds/b726215b8b978e1d85257af5c2fd0a6fb8e116fd/
+gs://container-diff/builds/b726215b8b978e1d85257af5c2fd0a6fb8e116fd/container-diff-darwin-amd64.sha256
+gs://container-diff/builds/b726215b8b978e1d85257af5c2fd0a6fb8e116fd/container-diff-linux-amd64
+gs://container-diff/builds/b726215b8b978e1d85257af5c2fd0a6fb8e116fd/container-diff-linux-amd64.sha256
+gs://container-diff/builds/b726215b8b978e1d85257af5c2fd0a6fb8e116fd/container-diff-windows-amd64.exe.sha256
+```
+
+The artifacts built and stored are roughly equivalent to the `make cross` target in our Makefile.
+
+The `cloudbuild.yaml` at the project root is used to build these artifdacts.
+
+## Release Builds
+
+When a new tag is pushed to Github, a second Container Builder pipeline is executed to build and upload release binaries.
+These are stored in another GCS location, in the same bucket.
+These artifacts are named via the git TAG name.
+
+```shell
+$ gsutil ls gs://container-diff/
+gs://container-diff/builds/
+gs://container-diff/latest/
+gs://container-diff/v0.2.0/
+gs://container-diff/v0.4.0/
+gs://container-diff/v0.4.1/
+gs://container-diff/v0.5.0/
+```
+
+A second, `latest` location is setup as an alias to the latest release.
+This upload and aliases is handled automatically via the `cloudbuild-release.yaml` file located at the project root.
+
+## Release Instructions
+
+To perform a release, follow these steps:
+
+1. Select the `commit` to create the release at, preferably from the `master` branch.
+2. Create a new git `tag` and matching Github `release`, pointing to this commit.  
+  This can be done either through the UI or CLI.
+3. Write a descriptive release page.  
+  You can use the notes from the last release to help seed the template.
+3. Wait for the Container Builder release build to complete.  
+ You can follow this in the [UI](https://cloud.google.com/gcr/triggers).
+4. Mirror the release artifacts to the Github release page.  
+ (Download them from GCS and re-upload them to Github).