Add documentation on how to use kubeconform as a Github Action

Author: yannh

Readme.md

@@ -193,6 +193,34 @@ $ ./scripts/openapi2jsonschema.py https://raw.githubusercontent.com/aws/amazon-s
 JSON schema written to trainingjob-sagemaker-v1.json
 ```
 
+### Usage as a Github Action
+
+Kubeconform is publishes Docker Images to Github's new Container Registry, ghcr.io. These images
+can be used directly in a Github Action, once logged in using a [_Github Token_](https://github.blog/changelog/2021-03-24-packages-container-registry-now-supports-github_token/).
+
+Example:
+```
+name: kubeconform
+on: push
+jobs:
+  kubeconform:
+    runs-on: ubuntu-latest
+    steps:
+      - name: login to Github Packages
+        run: echo "${{ github.token }}" | docker login https://ghcr.io -u ${GITHUB_ACTOR} --password-stdin
+      - uses: actions/checkout@v2
+      - uses: docker://ghcr.io/yannh/kubeconform:master
+        with:
+          entrypoint: '/kubeconform'
+          args: "-summary -output json kubeconfigs/"
+```
+
+_Note on pricing_: Kubeconform relies on Github Container Registry which is currently in Beta. During that period,
+[bandwidth is free](https://docs.github.com/en/packages/guides/about-github-container-registry). After that period,
+bandwidth costs might be applicable. Since bandwidth from Github Packages within Github Actions is free, I expect
+Github Container Registry to also be usable for free within Github Actions in the future. If that were not to be the
+case, I might publish the Docker image to a different platform.
+
 ### Speed comparison with Kubeval
 
 Running on a pretty large kubeconfigs setup, on a laptop with 4 cores: