Add note in contributing guideline about types of contributions

[alamb]

As the community grows, I am trying to help it scale and not be bottlenecked on review by the maintainers.

Ideally by the time one of the maintainers sees a PR it is more or less ready to go (though maybe that is wishful thinking)

Rendered version here: https://github.com/alamb/arrow-rs/blob/alamb/review_plea/CONTRIBUTING.md

[codecov-commenter]

Codecov Report

Merging #1396 (545377c) into master (ab41c25) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1396      +/-   ##
==========================================
- Coverage   83.10%   83.10%   -0.01%     
==========================================
  Files         181      181              
  Lines       53244    53244              
==========================================
- Hits        44247    44246       -1     
- Misses       8997     8998       +1     

Impacted Files	Coverage Δ
arrow/src/array/transform/mod.rs	86.20% <0.00%> (-0.23%)	⬇️
arrow/src/datatypes/datatype.rs	66.80% <0.00%> (+0.39%)	⬆️

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