adding authors.rst, contributing.rst and readme.rst to docs.

Also changed 'CONTRIBUTORS.txt' to 'AUTHORS.txt' following the example of https://github.com/audreyr/cookiecutter-pypackage
and also django (they use only 'AUTHORS' there, but I the idea is the same).
http version is more easy to new git users

$ git clone https://github.com/your_username_here/{{ cookiecutter.repo_name }}.git