add CONTRIBUTING.md (#84)

CONTRIBUTING.md

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+# Contributing
+
+## General
+
+A good general description on how to develop a GitHub JavaScript action can be found [here](https://docs.github.com/en/actions/creating-actions/creating-a-javascript-action).
+
+## Developing locally
+
+### Prerequisites
+
+- Node 20
+
+### Installing dependencies
+
+`npm install`
+
+### Building the action
+
+`npm run build`
+
+Updates the contents of the `dist` folder, which then _needs to be committed_.
+
+### Formatting source files
+
+`npm run format`
+
+### Checking formatting of source files
+
+`npm run format-check`
+
+### Linting source files
+
+`npm run lint`
+
+### Running tests
+
+`npm run test`
+
+### Running all of the above sequentially
+
+`npm run all`
+
+## Adding a new input parameter
+
+The following places need to be updated
+
+- action.yml
+- src/main.ts (`getCheckReleaseArguments()`)
+- dist/index.js (automatically generated)
+- README.md