CONTRIBUTING.rst: Updating CONTRIBUTING guidelines

This is almost a complete rewrite of the CONTRIBUTING guide.

  * The patch submission guidelines have become less ambiguous and more strict

  * Some general restructuring and reordering of the file took place

  * The codeing guidelines have changed completely.

    o There is much less room for ambiguity here now
    o More emphasis on consistency in the codebase
    o Added some more abstract points which should be considered
      when writing and reviewing patches

  * The policy on public/private symbols has been greatly clarified

  * Added new section about adding new core plugins and what needs
    to be done as a consequence of that