Convert README from MD to RST

With sphinx we'll have to write the documentation in restructuredtext
anyway, so we might use it for the README as well.

REDAME.rst

@@ -0,0 +1,99 @@
+.. image:: https://travis.ci.org/pyradius/pyrad3.svg?branch=master
+    :target: https://travis-ci.org/pyradius/pyrad3
+.. image:: https://coveralls.io/repos/github/pyradius/pyrad3/badge.svg?branch=master
+    :target: https://coveralls.io/github/pyradius/pyrad3?branch=master
+.. image:: https://img.shields.io/pypi/v/pyrad3.svg
+    :target: https://pypi.python.org/pypi/pyrad3
+.. image:: https://img.shields.io/pypi/pyversions/pyrad.svg
+    :target: https://pypi.python.org/pypi/pyrad3
+.. image:: https://img.shields.io/pypi/dm/pyrad.svg
+    :target: https://pypi.python.org/pypi/pyrad3
+
+Introduction
+============
+
+pyrad3 is an implementation of a RADIUS client/server as described in RFC2865.
+It takes care of all the details like building RADIUS packets, sending them and
+decoding responses.
+
+This is the successor of pyrad. It is written in python3 only (python3.7+) and
+has an improved API.
+
+Requirements & Installation
+===========================
+
+pyrad requires Python 3.7 or later. Currently no external dependencies are
+needed to use pyrad3:: cli
+
+  # Install from pypi
+  pip install pyrad3
+
+  # Install from the repository
+  git clone https://github.com/pyradius/pyrad3
+  pip install .
+
+  # Installing with poetry
+  git clone https://github.com/pyradius/pyrad3
+  poetry install --no-dev # for usage; without optional dev dependencies
+  poetry install # for development
+
+Development
+===========
+
+pyrad3 usese quite a lot of linters and development tools:
+
+- coveralls
+- black
+- poetry
+- pytest
+  - pytest-black
+  - pytest-cov
+  - pytest-flake8
+  - pytest-isort
+  - pytest-mypy
+  - pytest-pylint
+- tox
+- sphinx
+
+This project uses poetry as it packaging and dependency management tool. The
+configuration is done via `pyproject.toml`. To install this package with `pip
+install`, a pip version greater than `19.0` is required.
+
+When poetry is installed, the development dependencies can be installed with
+`poetry install --no-root`. Omitting the `--no-root` parameter will additionally
+install pyrad3 itself.
+
+
+with nix
+--------
+
+
+Generating the documentation
+----------------------------
+
+Sphinx is used for the documentation. It can be either run directly with
+` ` or with `make` (requires `gnumake` to be installed.)
+
+TODO: no documentation yet.
+
+Running tests locally
+---------------------
+
+pyrad3 comes with a tox configuration (check the `[tool.tox]` section in
+./pyproject.) Tox will set up test environments for all supported python
+versions and run them automatically.
+
+<!-- TODO: is this correct? -->
+It is necessary that the python versions are installed otherwise tox will fail.
+To execute a test with a single python version run `tox -e $version`, e.g.
+`tox -e py38`.
+
+License
+=======
+
+pyrad3 is distributed under the terms of both the MIT License and the Apache
+License (Version 2.0).
+
+See `LICENSE-APACHE ./LICENSE-APACHE` and `LICENSE-MIT ./LICENSE-MIT` for
+details.
+