Create a basic CONTRIBUTING.md (#3333)

This PR proposes a CONTRIBUTING.md giving some basic guidance on
contributing to KeY.
It is based on the [wiki
page](https://git.key-project.org/key/key/-/wikis/CONTRIBUTING).
I decided to not mention the outdated gitlab.com mirrors (related:
KeYProject/key-docs#15).

Resolves #1593 (see that issue for more discussion).

CONTRIBUTING.md

@@ -0,0 +1,24 @@
+# Contributing to KeY
+
+KeY is open source and you are welcome to report issues and contribute code.
+
+## Reporting issues
+
+If you observe a bug or other issue, please report it in the [issues section](https://github.com/KeYProject/key/issues).
+If you want to report an issue privately, send an email to [email protected].
+
+## Contributing code
+
+You can fork this repository into your namespace, create branches and develop as you like.
+
+If you have fixed an issue or implemented a new feature, feel free to [open a pull request](https://github.com/KeYProject/key/compare).
+Pull requests are assessed automatically by running tests, checking for formatting errors and running static source checkers
+before they are reviewed by the core team.
+
+If you plan to implement major new functionality, we suggest first getting in contact with the KeY team (open an issue). We are glad to help and to guide.
+
+More detailed information is available in the [KeY Developer Documentation](https://keyproject.github.io/key-docs/devel/). In particular, please make sure your code adheres to the [Java Code Conventions](https://keyproject.github.io/key-docs/devel/CodingConventions/).
+
+### Licensing
+
+KeY is published under the [GPLv2](https://www.gnu.org/licenses/old-licenses/gpl-2.0.html.en) (no later versions). By opening a PR, you are licensing your contribution under the GPLv2 for inclusion in KeY.