Add first draft of CONTRIBUTING.md

[jsvine]

Per suggestions in #387.

[codecov]

Codecov Report

Merging #428 (9ffe646) into develop (1328a00) will not change coverage.
The diff coverage is n/a.

❗ Current head 9ffe646 differs from pull request most recent head e98180e. Consider uploading reports for the commit e98180e to get more accurate results

@@           Coverage Diff            @@
##           develop     #428   +/-   ##
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  Coverage    98.28%   98.28%           
========================================
  Files           10       10           
  Lines         1227     1227           
========================================
  Hits          1206     1206           
  Misses          21       21           

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[samkit-jain]

Looks good!

[jsvine]

Thanks!