Update usage.rst
Update README.rst

Added required configuration steps.
Merge pull request #2 from programmdesign/patch-1

Update usage.rst
Merge pull request #3 from programmdesign/patch-2

Update README.rst
Added utf-8 headers
Merge branch 'master' of github.com:martinblech/django-critical
Added import of unicode literals
Fixed #1 and added setting for encoding
Enhanced handling of ajax calls