Update README.rst

[Nishant-Ingle]

README.rst: Change coala-bears usage link

This fix ensures that when user clicks on 'usage', he/she is redirected to coala user documentation page.

Closes #2846

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[utkarsh2102]

Should it not link to http://docs.coala.io/en/latest/Users/coala_quickstart.html#usage instead?