Document release process (kubernetes-sigs#768)

Signed-off-by: Chuck Ha <[email protected]>

Author: chuckha

docs/developer/releasing.md

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+# Releasing
+
+## Output
+
+### Expected artifacts
+
+1. A container image of the shared cluster-api controller manager
+2. A git tag for providers to use
+
+### Artifact locations
+
+1. The container image is found in the registry `gcr.io/k8s-cluster-api` with an
+   image name of `cluster-api-controller` and a tag that matches the release
+   version. For example, in the `0.0.0-alpha.4` release, the container image
+   location is `gcr.io/k8s-cluster-api/cluster-api-controller:0.0.0-alpha.4`
+
+## Process
+
+1. Create a pull request that contains two changes:
+   1. Change [the cluster api controller manager image
+   tag][managerimg] from `:latest` to whatever version is being released
+   2. Change the `CONTROLLER_IMAGE` variable in the [Makefile][makefile] to the
+      version being released
+2. Get the pull request merged
+3. From the commit in step 1 (that is now in the master branch), build and push
+   the container image with `make docker-push`
+4. Create a tag from this same commit and push the tag to the github repository
+5. Revert the commit made in step 1
+6. Open a pull request with the revert change
+7. Get that pull request merged
+8. Create a release in github based on the tag created above
+9. Manually create the release notes by going through the merged PRs since the
+   last release
+
+[managerimg]: https://github.com/kubernetes-sigs/cluster-api/blob/fab4c07ea9fb0f124a5abe3dd7fcfffc23f2a1b3/config/default/manager_image_patch.yaml
+[makefile]: https://github.com/kubernetes-sigs/cluster-api/blob/fab4c07ea9fb0f124a5abe3dd7fcfffc23f2a1b3/Makefile
+
+### Permissions
+
+Releasing requires a particular set of permissions.
+
+* push access to the gcr bucket
+* tag push access to the github repository
+* release creation