Use reStructuredText as file format for README

rst seems to be cooler than md, and is the only format accepted by pypi,
and is accepted by github as well.

There is an intro to rst at:
http://docutils.sourceforge.net/docs/user/rst/quickref.html

setup.py

@@ -17,6 +17,7 @@ setup(
     description='Tool to download pictures (or videos) and captions from Instagram, from a given '
                 'set of profiles (even if private), from your feed or from all followees of a '
                 'given profile.',
+    long_description=open('README.rst').read(),
     install_requires=['requests>=2.4'],
     python_requires='>=3.3',
     entry_points={'console_scripts': ['instaloader=instaloader:main']},