Draft contributors.md update (#97)

* Draft contributors.md update

This is a proposed updated to the contributors.md document inspired by
the contributing guidelines of the actions/cache project [1]. There are
some differences, most notable the inclusion of "Development Details"
which explains and motivates the development tools (to an extend).

--
1. https://github.com/actions/cache/blob/eed9cfe64d00dd64cbb36bec915d41dd6fab9f6c/CONTRIBUTING.md

* Split Contributing Guidelines and Development Handbook

Split the "Contributing Guidelines" currently found in
docs/contributors.md into a CONTRIBUTING.md file, containing the
Contributing Guidelines, and a docs/development.md file, containing
details on how the development/testing of this project works.

Author: ericcornelissen

CONTRIBUTING.md

@@ -0,0 +1,26 @@
+# Contributing Guidelines
+
+Hi there! We're thrilled that you'd like to contribute to this project. Your help is essential for keeping it great.
+
+Contributions to this project are [released](https://help.github.com/articles/github-terms-of-service/#6-contributions-under-repository-license) to the public under the [project's open-source license](../LICENSE).
+
+## Submitting a Pull Request
+
+1. [Fork](https://github.com/actions/labeler/fork) and clone the repository
+1. Configure and install the dependencies: `npm install`
+1. Create a new branch: `git checkout -b my-branch-name`
+1. Make your change and build the action using `npm run build`
+1. Push to your fork and [submit a pull request](https://github.com/actions/labeler/compare)
+1. Pat your self on the back and wait for your pull request to be reviewed and merged.
+
+Here are a few things you can do that will increase the likelihood of your pull request being accepted:
+
+- Follow the coding style used in this project. You can do this by running `npm run format`.
+- Keep your change as focused as possible. If there are multiple changes you would like to make that are not dependent upon each other, consider submitting them as separate pull requests.
+- Write [good commit messages](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
+
+## Resources
+
+- [How to Contribute to Open Source](https://opensource.guide/how-to-contribute/)
+- [Using Pull Requests](https://help.github.com/articles/about-pull-requests/)
+- [GitHub Help](https://help.github.com)

docs/contributors.md

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+# Development Handbook
+
+This project is written in [TypeScript](https://www.typescriptlang.org/), a typed variant of JavaScript, and we use [Prettier](https://prettier.io/) to get a consistent code style.
+
+Because of how GitHub Actions are run, the source code of this project is transpiled from TypeScript into JavaScript. The transpiled code (found in `lib/`) is subsequently compiled using [NCC](https://github.com/vercel/ncc/blob/master/readme.md) (found in `dist/`) to avoid having to include the `node_modules/` directory in the repository.