[DOCS] Remove docker reference in contributing

[emdupre]

(Re-)Closes #51.

Changes proposed in this pull request:

• Documentation only update— removes docker reference in RTD contributing docs

[codecov]

Codecov Report

Merging #68 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #68   +/-   ##
=======================================
  Coverage   28.06%   28.06%           
=======================================
  Files          25       25           
  Lines        1372     1372           
=======================================
  Hits          385      385           
  Misses        987      987

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