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).
We are not using `tox` in project. Also `flake8` should be able to run without passing folder names, if have something like this in `setup.cfg`

```
[flake8]
max-line-length = 120
exclude = .tox,.git,*/migrations/*,*/static/*,docs
```