fixed documentation link in README

[christy-thompson]

Fixed the documentation link in the README file (see branch documenter).

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.44%. Comparing base (fb66c13) to head (70b0cd2).
Report is 6 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #195       +/-   ##
===========================================
+ Coverage   62.60%   75.44%   +12.83%     
===========================================
  Files           5        5               
  Lines         845      908       +63     
===========================================
+ Hits          529      685      +156     
+ Misses        316      223       -93     

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