Add a code of conduct

[sgugger]

What does this PR do?

This PR adds a code of conduct to the project, inspired by the Contributor Covenant. To make it clearly visible it also adds:

• a badge that displays under Transformers on the main README.
• a link in the contributing guide.

[codecov]

Codecov Report

Merging #7433 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7433      +/-   ##
==========================================
- Coverage   76.85%   76.84%   -0.02%     
==========================================
  Files         181      181              
  Lines       35800    35800              
==========================================
- Hits        27513    27509       -4     
- Misses       8287     8291       +4     

Impacted Files	Coverage Δ
src/transformers/tokenization_t5.py	66.34% <0.00%> (-28.85%)	⬇️
src/transformers/file_utils.py	83.37% <0.00%> (+0.24%)	⬆️
src/transformers/tokenization_mbart.py	96.82% <0.00%> (+39.68%)	⬆️

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[LysandreJik]

LGTM, great addition. Thanks for adding it!

[thomwolf]

Yes!

[julien-c]

(before merging, let's check out the thread we had internally on this)