Explain how to use tox-gh-actions with tox's requires configuration

[ymyzk]

Description

Improve README to cover a case reported in #102

Expected Behavior

Users can tell how to use tox's require + tox-gh-actions more easily.

[codecov-commenter]

Codecov Report

Merging #121 (1e7095c) into master (2c268f8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #121   +/-   ##
=======================================
  Coverage   81.33%   81.33%           
=======================================
  Files           2        2           
  Lines         150      150           
  Branches       39       39           
=======================================
  Hits          122      122           
  Misses         17       17           
  Partials       11       11           

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