Add documentation on how to contribute

Author: sebastianbergmann

.github/CONTRIBUTING.md

@@ -1 +1,75 @@
-Please refer to [https://github.com/sebastianbergmann/phpunit/blob/main/CONTRIBUTING.md](https://github.com/sebastianbergmann/phpunit/blob/main/.github/CONTRIBUTING.md) for details on how to contribute to this project.
+# Contributing to `phpunit/php-code-coverage`
+
+## Welcome!
+
+We look forward to your contributions! Here are some examples how you can contribute:
+
+* [Report a bug](https://github.com/sebastianbergmann/php-code-coverage/issues/new)
+* [Send a pull request to fix a bug](https://github.com/sebastianbergmann/php-code-coverage/pulls)
+
+Please do not send pull requests that expand the scope of this project (see below).
+
+
+## Any contributions you make will be under the BSD-3-Clause License
+
+When you submit code changes, your submissions are understood to be under the same [BSD-3-Clause License](https://github.com/sebastianbergmann/php-code-coverage/blob/main/LICENSE) that covers the project. By contributing to this project, you agree that your contributions will be licensed under its BSD-3-Clause License.
+
+
+## Write bug reports with detail, background, and sample code
+
+[This is an example](https://github.com/sebastianbergmann/phpunit/issues/4376) of a bug report I wrote, and I think it's not too bad.
+
+In your bug report, please provide the following:
+
+* A quick summary and/or background
+* Steps to reproduce
+    * Be specific!
+    * Give sample code if you can.
+* What you expected would happen
+* What actually happens
+* Notes (possibly including why you think this might be happening, or stuff you tried that didn't work)
+
+Please post code and output as text ([using proper markup](https://guides.github.com/features/mastering-markdown/)). Do not post screenshots of code or output.
+
+
+## Workflow for Pull Requests
+
+1. Fork the repository.
+2. Create your branch from the oldest branch that is affected by the bug you plan to fix.
+3. Implement your change and add tests for it.
+4. Ensure the test suite passes.
+5. Ensure the code complies with our coding guidelines (see below).
+6. Send that pull request!
+
+Please make sure you have [set up your username and email address](https://git-scm.com/book/en/v2/Getting-Started-First-Time-Git-Setup) for use with Git. Strings such as `silly nick name <root@localhost>` look really stupid in the commit history of a project.
+
+We encourage you to [sign your Git commits with your GPG key](https://docs.github.com/en/github/authenticating-to-github/signing-commits).
+
+
+## Development
+
+This project uses [PHPUnit](https://phpunit.de/) for testing:
+
+```shell
+./vendor/bin/phpunit
+```
+
+This project uses [PHPStan](https://phpstan.org/) for static analysis:
+
+```shell
+./tools/phpstan
+```
+
+This project uses [PHP-CS-Fixer](https://cs.symfony.com/) to enforce coding guidelines:
+
+```shell
+./tools/php-cs-fixer fix
+```
+
+The commands shown above require an autoloader script at `vendor/autoload.php`. This can be generated like so:
+
+```shell
+./tools/composer dump-autoload
+```
+
+Please understand that we will not accept a pull request when its changes violate this project's coding guidelines or break the test suite.