Add link to documentation

[jugmac00]

This link will be shown at https://pypi.org/project/isort/ and makes it easier to find the documentation.

[codecov-io]

Codecov Report

Merging #1153 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop   #1153   +/-   ##
=======================================
  Coverage     95.8%   95.8%           
=======================================
  Files           49      49           
  Lines         3549    3549           
=======================================
  Hits          3400    3400           
  Misses         149     149

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