README: Add repology badge

[luzpaz]

This badge will display all the downstream repositories that carry GitPython and the version number.

Edit: To see a preview go to https://github.com/luzpaz/GitPython/tree/README-repology-badge#development-status

Edit2: fixed the above link

[codecov-io]

Codecov Report

Merging #835 into master will decrease coverage by 0.84%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #835 +/- ##
==========================================
- Coverage 94.79% 93.94% -0.85% 
==========================================
Files 59 59 Lines 9600 9386 -214 ==========================================
- Hits 9100 8818 -282 - Misses 500 568 +68

Impacted Files	Coverage Δ
git/compat.py	40.74% <0%> (-24.08%)	⬇️
git/test/lib/asserts.py	61.53% <0%> (-7.7%)	⬇️
git/test/test_git.py	96.53% <0%> (-2.36%)	⬇️
git/objects/tree.py	59.06% <0%> (-1.51%)	⬇️
git/refs/log.py	93.07% <0%> (-1.29%)	⬇️
git/refs/head.py	97.8% <0%> (-1.12%)	⬇️
git/test/test_fun.py	97.84% <0%> (-1.1%)	⬇️
git/diff.py	98.26% <0%> (-0.9%)	⬇️
git/test/test_commit.py	99.13% <0%> (-0.87%)	⬇️
git/test/lib/helper.py	91.27% <0%> (-0.86%)	⬇️
... and 28 more

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[Byron]

Thanks a lot for your contribution :)!