Improve the documentation and README.md

[marvinpfoertner]

• Write introductory text/project summary
• Update information in setup.cfg (e.g. keywords, classifiers)
• Restructure documentation
• Document how to build without Cython
• Replace :math:`A'` in docs by :math:`A^+` for updated matrices and :math:`A^-` for downdated matrices
• Add reference to "scipy.linalg.cholesky" as a source for column-major Cholesky decompositions
• Make list of exceptions more concrete
• Move theoretical derivation of update algorithms from Notes section to own documentation page and reference.
• Add GitHub citation function