Add labels to cherry pick scripts + writeup

[alamb]

re #292

Rationale for this change

I am trying to use labels to make it clear which PRs has been backported and what has not been backported.

This PR is a step towards my final goal of #292, which will be to automate the invocation of backporting in a github action.

What changes are included in this PR?

1. The cherry-pick script adds labels now
2. Adds documentation about how this works

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #409 (9e66463) into master (0a16574) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #409      +/-   ##
==========================================
- Coverage   82.62%   82.62%   -0.01%     
==========================================
  Files         162      162              
  Lines       44479    44479              
==========================================
- Hits        36751    36750       -1     
- Misses       7728     7729       +1     

Impacted Files	Coverage Δ
parquet/src/encodings/encoding.rs	94.85% <0.00%> (-0.20%)	⬇️

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