Fixing README.rst to render correctly on PyPI.

Main changes:
- Using named links instead of anonymous
- Using an absolute instead of project relative link for
  CONTRIBUTING
- Changing duplicated link text so that they differ
- Lining up :target: with the line above for embedded img's

H/T to
https://github.com/coagulant/coveralls-python/blob/master/README.rst
for guidance.