DOC: Add contributing page to RTD

[mgxd]

• Change CONTRIBUTING to rst.
• Add contributing section to RTD

[codecov-commenter]

Codecov Report

Merging #130 (38e0407) into master (3c763d4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #130   +/-   ##
=======================================
  Coverage   98.88%   98.88%           
=======================================
  Files          12       12           
  Lines        1081     1081           
  Branches      138      138           
=======================================
  Hits         1069     1069           
  Misses          6        6           
  Partials        6        6           

Flag	Coverage Δ
travis	98.88% <ø> (ø)

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