Add documentation for pydot-ng

[prmtl]

Documentation should be hosted on Github Pages (github.io) or on Read The Docs. Probably Read The Docs will be better since it was created for such tasks.

Things that need to be done:

• choose hosting place
• integrate with Sphinx (do we want to choose anything else?)
• ...

Some things that need to be covered:

• dev guide
• how to replace pydot with pydot-ng
• some short info about DOT language (see for example [88] Guidelines for Node label formatting #24)
• integration with networkx
• ...

[prmtl]

Skeleton on ReadTheDocs: http://pydot-ng.readthedocs.org/en/add_docs/
Changes in add_docs branch