docs: Add CONTRIBUTING.md

Author: komaeda

CONTRIBUTING.md

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+## Contributing to Rustlings
+
+First off, thanks for taking the time to contribute!! ❤️
+
+### Quick Reference
+
+I want to...
+
+_update an outdated exercise! ➡️ [open a Pull Request](#prs)_
+
+_report a bug! ➡️ [open an Issue](#issues)_
+
+_fix a bug! ➡️ [open a Pull Request](#prs)_
+
+_implement a new feature! ➡️ [open an Issue to discuss it first, then a Pull Request](#issues)_
+
+<a name="issues"></a>
+### Issues
+
+You can open an issue [here](https://github.com/rust-lang/rustlings/issues/new).
+If you're reporting a bug, please include the output of the following commands:
+
+- `rustc --version`
+- `rustlings --version`
+- `ls -la`
+- Your OS name and version
+
+<a name="prs"></a>
+### Pull Requests
+
+Opening a pull request is as easy as forking the repository and committing your
+changes. There's a couple of things to watch out for:
+
+#### Write correct commit messages
+
+We follow the [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0-beta.4/)
+specification, because it makes it easier to generate changelogs automatically.
+This means that you have to format your commit messages in a specific way. Say
+you're working on adding a new exercise called `foobar1.rs`. You could write
+the following commit message:
+
+```
+feat: Add foobar1.rs exercise
+```
+
+If you're just fixing a bug, please use the `fix` type:
+
+```
+fix(verify): Make sure verify doesn't self-destruct
+```
+
+The scope within the brackets is optional, but should be any of these:
+
+- `installation` (for the installation script)
+- `cli` (for general CLI changes)
+- `verify` (for the verification source file)
+- `watch` (for the watch functionality source)
+- `run` (for the run functionality source)
+- `EXERCISENAME` (if you're changing a specific exercise, or set of exercises,
+  substitute them here)
+
+When the commit also happens to close an existing issue, link it in the message
+body:
+
+```
+fix: Update foobar
+
+closes #101029908
+```
+
+If you're doing simple changes, like updating a book link, use `chore`:
+
+```
+chore: Update exercise1.rs book link
+```
+
+If you're updating documentation, use `docs`:
+
+```
+docs: Add more information to Readme
+```
+
+If, and only if, you're absolutely sure you want to make a breaking change
+(please discuss this beforehand!), add an exclamation mark to the type and
+explain the breaking change in the message body:
+
+```
+fix!: Completely change verification
+
+BREAKING CHANGE: This has to be done because lorem ipsum dolor
+```
+
+#### Pull Request Workflow
+
+Once you open a Pull Request, it may be reviewed or labeled (or both) until
+the maintainers accept your change. Then, [bors](https://github.com/bors) will
+run the test suite with your changes and if it's successful, automatically
+merge it in!