Update README.rst with info on using config file

[alexandrebarbaruiva]

Closes: #195

[codecov-commenter]

Codecov Report

Merging #198 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #198   +/-   ##
=======================================
  Coverage   67.15%   67.15%           
=======================================
  Files           6        6           
  Lines         344      344           
=======================================
  Hits          231      231           
  Misses        113      113           

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Thanks !