[MRG] Add "good PR" examples

[standage]

Some small additions to the dev docs for new contributors. Closes #464.

Ready for review! Although our docs build seems to be choking on a sphinx dependency...

• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov-io]

Codecov Report

Merging #1848 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1848   +/-   ##
======================================
  Coverage    0.01%   0.01%           
======================================
  Files          78      78           
  Lines       33208   33208           
  Branches      434     434           
======================================
  Hits            5       5           
  Misses      33203   33203

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