Update and add general documentation

A general description as well as instructions for installation and
building documentation are added.

README.md

-Implementation of the Jackknife algorithm to estimate the error of the self energy in DGA calculations
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+# Jackknife
+
+Implementation of jackknife resampling applied to abinitio DGA calculations to estimate errors
+and covariance of self-energies and susceptibilities.
+
+For more info and documentation see: https://patrickkappl.gitlab.io/jackknife/
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doc/source/index.rst

 Welcome to jackknife's documentation!
-=====================================
+=========================================
+
+.. TODO: note the modular nature of the code which allows to easily apply it to things other than
+   ADGA
+
+This code applies the method of jackknife resampling to abinito DGA calculations. Thus it allows to
+estimate e.g. the standard error of the mean or the covariance matrix of the DGA self-energy and
+susceptibilities.
 
-.. TODO: add the following stuff
 
-Short description:
 Installation:
+-------------
+`jackknife` is written in Python 3 and requires the following packages:
+
+   - NumPy (1.16.2)
+   - h5py (1.10.2)
+   - configparser (3.7.4)
+
+The numbers in parenthesis are not the minimum required version but just one set of versions that
+are tested and work. In addition
+
+   - w2dynamics_ and
+   - ADGA_
+
+must also be installed (instructions on how to do that are provided in their respective
+documentation). ADGA is the program that actually calculates the self-energies and susceptibilities.
+w2dynamics is a DMFT code used to calculate 1- and 2-particle Green's functions, which are the input
+quantities of ADGA.
+
+.. _ADGA: https://github.com/AbinitioDGA/ADGA
+.. _w2dynamics: https://github.com/w2dynamics/w2dynamics
+
+
+Building documentation:
+-----------------------
+The documentation you are currently reading is built with Sphinx_. It uses Pyhton's docstrings and
+reStructuredText. To build it on your local machine you need to install Sphinx (e.g. with conda) 
+and execute the following commands::
+
+   cd .../jackknife/doc
+   make html
+
+The generated HTML files can be found in *.../jackknife/doc/build/html*. The main page is called
+*index.html*.
+
+.. _Sphinx: http://www.sphinx-doc.org/en/master/
+
+.. TODO: add the following stuff
+
 Usage/Tutorial:
-How to build documentation:
 Code Documentation:
 
+
 List of modules
 ---------------
 .. toctree::
@@ -21,6 +64,7 @@ List of modules
 .. TODO: add something like a getting started with installation, requirements, config description
    and so on
 
+
 Indices
 -------
 * :ref:`genindex`