Use README.md instead README.rst

[scaleoutsean]

# python3.5 setup.py build
Traceback (most recent call last):
  File "setup.py", line 20, in <module>
    with open(os.path.join(here, 'README.rst'), encoding='utf-8') as f:
  File "/usr/local/lib/python3.5/codecs.py", line 895, in open
    file = builtins.open(filename, mode, buffering)
FileNotFoundError: [Errno 2] No such file or directory: '/data/solidfire-python-sdk/solidfire-sdk-python-master/README.rst'

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This change is