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 I decided to not mention the outdated gitlab.com mirrors (related: https://github.com/KeYProject/key-docs/issues/15).

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

[codecov[bot]]

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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