Switch to README.md for the description on PyPi

[tompollard]

Currently we have a two readme files: README.rst and README.md

README.rst file (which is used for the description on PyPI) contains errors. e.g., it suggests using the following columns as an argument, but these columns do not appear in the data:

columns = ['age','bili','albumin','ast','platelet','protime',
       'ascites','hepato','spiders','edema','sex', 'trt']

This pull request:

• switches to using README.md on PyPI
• removes the unnecessary README.rst file
• bumps the package version to '0.7.11'