Update CONTRIBUTING.md

[perlun]

Fixed the headers. They were rendering incorrectly on GitHub for some weird reason; I copied the strings and rewrote the ## characters at the beginning and now they render as they should.

Rendered version

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "patch-1" [email protected]:perlun/distribution.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[perlun]

@GordonTheTurtle Done. Bonus points for the excellent instructions on how to clone my fork, amend the commit etc. 👍 Makes it effortless really.

[codecov]

Codecov Report

Merging #2484 into master will decrease coverage by 9.82%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #2484      +/-   ##
=========================================
- Coverage   61.42%   51.6%   -9.83%     
=========================================
  Files         128     128              
  Lines       11685   11685              
=========================================
- Hits         7178    6030    -1148     
- Misses       3615    4903    +1288     
+ Partials      892     752     -140

Impacted Files	Coverage Δ
registry/storage/driver/gcs/gcs.go	0.4% <0%> (-68.94%)	⬇️
registry/storage/driver/oss/oss.go	0.56% <0%> (-57.23%)	⬇️
registry/storage/driver/s3-aws/s3.go	4.57% <0%> (-56.23%)	⬇️
registry/storage/driver/s3-goamz/s3.go	0.5% <0%> (-51.4%)	⬇️
registry/client/transport/transport.go	69.69% <0%> (-9.1%)	⬇️

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[thaJeztah]

ah, thanks! guess there was a different white-space character in between (a tab?)

LGTM

[thaJeztah]

ping @stevvooe for the merge (I don't have permissions to merge due to codecov complaining 😅)

[stevvooe]

LGTM

codecov is the worst.