Update release notes and add 'make release'

Author: thockin

Makefile

@@ -222,6 +222,15 @@ manifest-list: all-push
 	    --template $(REGISTRY)/$(BIN):$(VERSION)__OS_ARCH \
 	    --target $(REGISTRY)/$(BIN):$(VERSION)
 
+release:
+	if [ -z "$(TAG)" ]; then        \
+		echo "ERROR: TAG must be set"; \
+		false;                  \
+	fi
+	docker pull "$(BUILD_IMAGE)"
+	git tag -am "$(TAG)" "$(TAG)"
+	make manifest-list
+
 version:
 	echo $(VERSION)
 

RELEASING.md

@@ -2,36 +2,34 @@
 
 ## Tags
 
-First, see what has been tagged:
+First, pick the new tag.  Usually this means to see what has already been
+tagged, and pick the next release number.
 
 ```
 git tag
 ```
 
-Pick the next release number and tag it.
+## Log in
+
+Make sure you are logged into Google Cloud (to push to GCR).
 
 ```
-git tag -am v3.3.2 v3.3.2
+gcloud auth login
 ```
 
 ## Build and push to staging
 
 To build git-sync you need [docker buildx](https://github.com/docker/buildx).
 
-Make sure you are logged into Google Cloud (to push to GCR).
-
-```
-gcloud auth login
-```
-
 The following step will build for all platforms and push the container images
 to our staging repo (gcr.io/k8s-staging-git-sync).
 
 ```
-make manifest-list
+make release TAG="<tag you chose above>"
 ```
 
 This will produce output like:
+
 ```
 <...lots of output...>
 Successfully tagged gcr.io/k8s-staging-git-sync/git-sync:v3.3.2__linux_amd64