Create a basic CONTRIBUTING.md

[FliegendeWurst]

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I decided to not mention the outdated gitlab.com mirrors (related: KeYProject/key-docs#15).

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

[codecov]

Codecov Report

Merging #3333 (141204b) into main (1b1ddf1) will decrease coverage by 0.02%.
Report is 7 commits behind head on main.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #3333      +/-   ##
============================================
- Coverage     37.84%   37.82%   -0.02%     
+ Complexity    16883    16875       -8     
============================================
  Files          2052     2052              
  Lines        125439   125439              
  Branches      21215    21215              
============================================
- Hits          47469    47444      -25     
- Misses        72117    72141      +24     
- Partials       5853     5854       +1     

see 42 files with indirect coverage changes

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[wadoon]

Thank you for the effort.

Suggestions:

In general, a link to PRs should be given
also a link to the code guideline
The license should be made clearer, either to "GPL2-or-later" or "GPL2-only".
You may want to look in the new readme (used on [helmholtz.software] for Key), which states

Feel free to submit Pull Requests via Github. Pull Requests are assessed using an automatically tests, formatting and static source checkers. More guideline and documentation for the KeY development can be found under key-docs.