bug fix typo in README.rst

[rich0rd]

Correct spelling for read only properties is 'readonly'

[codecov]

Codecov Report

Merging #217 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #217   +/-   ##
=======================================
  Coverage   96.85%   96.85%           
=======================================
  Files          19       19           
  Lines        2705     2705           
=======================================
  Hits         2620     2620           
  Misses         85       85           

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[ziirish]

Thanks