Add recommendations for whitespace use to CONTRIBUTING.md

[ischoegl]

Changes proposed in this pull request

This PR adds recommendations for whitespace use to CONTRIBUTING.md. While these formatting recommendations were not pointed out in the style guidelines, they have been frequently provided in review comments

[codecov]

Codecov Report

Merging #1121 (ff8d030) into main (be95118) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1121   +/-   ##
=======================================
  Coverage   73.45%   73.45%           
=======================================
  Files         364      364           
  Lines       47882    47882           
=======================================
  Hits        35170    35170           
  Misses      12712    12712           

Impacted Files	Coverage Δ
src/thermo/PengRobinson.cpp	88.26% <100.00%> (ø)

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[speth]

This seems fine to me.