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).
"your GitHub username" is more explicit.

from "The Zen of Python", "...Explicit is better than implicit...".