README: add table with supported git features

[smola]

No description provided.

[codecov]

Codecov Report

Merging #384 into master will decrease coverage by 0.61%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #384      +/-   ##
==========================================
- Coverage   77.29%   76.68%   -0.62%     
==========================================
  Files         123      123              
  Lines        8753     8753              
==========================================
- Hits         6766     6712      -54     
- Misses       1226     1295      +69     
+ Partials      761      746      -15

Impacted Files	Coverage Δ
plumbing/transport/ssh/common.go	0% <0%> (-50.91%)	⬇️
plumbing/transport/ssh/auth_method.go	32.4% <0%> (-24.08%)	⬇️

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[mcuadros]

I will contribute to this PR this morning adding, "fully supported", 'basic supported"