Improve new contributor documentation

[terriko]

• Add imposter syndrome disclaimer (thanks @Niraj-Kamdar for suggesting this!)
• Improve flow (so people get told about virtualenvs, tests, and black before trying to push a PR)
• Add instructions for building documentation
• Fill in basic info on tests, virtualenv

[codecov-io]

Codecov Report

Merging #968 (a4f6159) into master (5c079a8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #968   +/-   ##
=======================================
  Coverage   85.03%   85.03%           
=======================================
  Files         169      169           
  Lines        2854     2854           
  Branches      311      311           
=======================================
  Hits         2427     2427           
  Misses        342      342           
  Partials       85       85           

Flag	Coverage Δ
longtests	85.03% <ø> (ø)

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