Add CONTRIBUTING.md

Author: sd109

CONTRIBUTING.md

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+# Contributing
+
+We welcome contributions and suggestions for improvements to this code base.
+Please check for relevant issues and PRs before opening a new one of your own.
+
+## Making a contribution
+
+### Helm template snapshots
+
+The CI in this repository uses the Helm
+[unittest](https://github.com/helm-unittest/helm-unittest) plugin's
+snapshotting functionality to check PRs for changes to the templated manifests.
+Therefore, if your PR makes changes to the manifest templates or values, you
+will need to update the saved snapshots to allow your changes to pass the
+automated tests. The easiest way to do this is to run the `helm unittest` command
+inside a docker container from the repo root.
+
+```
+docker run -i --rm -v $(pwd):/apps helmunittest/helm-unittest chart -u
+```
+
+where the `-u` option is used to update the existing snapshots.