initial sphinx docs

starts #69

- basic structure
- RTD theme
- installation
- quickstart (more to do)
- references
- added start page with license and links to repo and issues
- authors in docs: papers and repo contributors (to be discussed)

docs/source/installation.rst

@@ -0,0 +1,52 @@
+.. -*- coding: utf-8 -*-
+
+==============
+ Installation
+==============
+
+
+PROPKA 3 requires Python 3.6 or higher. Additional requirements are
+specified in the :file:`requirements.txt` file and automatically satisfied
+when installing with pip_.
+
+
+``pip``-based installation
+==========================
+
+The easiest way to install a release of PROPKA 3 is from the `PyPI archive`_ with the command
+
+.. code-block:: bash
+
+   pip install --upgrade propka
+
+   
+This installation will install the :mod:`propka` Python module and the
+:command:`propka3` executable script. As always, a virtual environment (e.g., via
+`virtualenv`_) is recommended when installing packages.
+
+Source-based installation
+=========================
+
+The source code can be installed by cloning the `repository`_ or
+unpacking from a source code archive and running
+
+.. code-block:: bash
+
+   pip install .
+
+in the source directory.
+
+For the purposes of testing or development, you may prefer to install
+PROPKA as an editable module via pip_ by running
+
+.. code-block:: bash
+
+   pip install -e .
+
+in the source directory.
+
+
+.. _pip: https://pip.pypa.io/
+.. _PyPI archive: https://pypi.org/project/PROPKA/
+.. _virtualenv: https://pypi.org/project/virtualenv/
+.. _repository: https://github.com/jensengroup/propka