Add Django 1.11 as a supported version to README.rst

[jorgebg]

After taking a look at the code, I think it already supports Django 1.11, am I wrong?

[codecov]

Codecov Report

Merging #60 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #60   +/-   ##
=======================================
  Coverage   63.89%   63.89%           
=======================================
  Files          12       12           
  Lines         781      781           
=======================================
  Hits          499      499           
  Misses        282      282

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[vdboor]

Thanks for paying attention! This is indeed the case.