Update readme.rst
Edit network analysis section of PyQGIS cookbook

Mainly fixed up some typos and grammar issues. Also added in details about what
values should be used in the QgsLineVectorLayerDirector class. The types
of input that are expected and what integer values are used with the
defaultDirection parameter.
Merge pull request #233 from sawcordwell/fixup_netan

Edit network analysis section of PyQGIS cookbook
Merge pull request #236 from egonzale/master

Update readme.rst
updated processing modeler entry