docs(contrib): Update contributing steps (#339)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

CONTRIBUTING.md

@@ -46,26 +46,42 @@ go test -v ./...
 
 Spin CLI uses [Swagger](https://swagger.io/) to generate the API client library for [Gate](https://github.com/spinnaker/gate).
 
-Spin CLI's `master` should be using Gate's `master` swagger definition. Similarly, each [spin release version](https://github.com/spinnaker/spin/tags) `v{major}.{minor}.{patch}` semver should match [Gate's tag](https://github.com/spinnaker/gate/tags) `version-{major}.{minor}`.
+Spin CLI's `master` branch should be using Gate's `master` swagger definition.
 
-Example:
-
-| Spin CLI version | Gate version   |
-| ---------------- | -------------- |
-| v1.17.3          | version-1.17.0 |
-| v1.17.2          | version-1.17.0 |
-| v1.17.1          | version-1.17.0 |
+Spin CLI's `release-{major}-{minor}.x` branch should be using Gate's
+corresponding `release-{major}-{minor}.x` swagger definition.
 
 To update the client library:
 
 - Use the Swagger Codegen to generate the new library and drop it into the spin project
+
   ```bash
-  GATE_REPO_PATH=PATH_TO_YOUR_GATE_REPO
+  # decide branch to update
+  branch=release-1.##.x
+
+  # check out appropriate Gate branch
+  # assuming Gate checked out in same parent directory as spin and up to date
+  cd ../gate
+  git checkout "$branch"
+
+  # generate Gate swagger client library branch
+  swagger/generate_swagger.sh
+
+  # check out appropriate Spin branch
+  cd ../spin
+  git checkout "$branch"
+
+  # set Swagger Codegen tool version
   SWAGGER_CODEGEN_VERSION=$(cat gateapi/.swagger-codegen/VERSION)
+
   rm -rf gateapi/ \
   && docker run -it \
-      -v "${GATE_REPO_PATH}/swagger/:/tmp/gate" \
+      -v "$PWD/../gate/swagger/:/tmp/gate" \
       -v "$PWD/gateapi/:/tmp/go/" \
       "swaggerapi/swagger-codegen-cli:${SWAGGER_CODEGEN_VERSION}" generate -i /tmp/gate/swagger.json -l go -o /tmp/go/
+
+  # create branch off $branch and PR changes
+  git checkout -b "$branch-swagger"
   ```
+
 - Commit the changes and open a PR.