add link to source docs in README.md (see #56)

[rozzzly]

See #56

[codecov-io]

Codecov Report

Merging #57 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop      #57   +/-   ##
========================================
  Coverage    89.32%   89.32%           
========================================
  Files           62       62           
  Lines          927      927           
  Branches       164      164           
========================================
  Hits           828      828           
  Misses          92       92           
  Partials         7        7

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