Doc update on coverage dependency

README.rst

   2.6, 2.7, 3.2, 3.3 and pypy
 - Only takes unified diff for input
 - Side by side mode has alignment problem for wide chars
-- Pull requests are very welcome (please run ``make test`` to verify)
+- Pull requests are very welcome, run ``make test`` to verify (required tool
+  *coverage* can be installed with ``pip install coverage``)
 
 See also
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