README: move to reST

* Moves README file to reST.  Amends location of workflow figures
  as per changes on yawlfoundation.org web site.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
README: addition of travis, coverage, pypi badges

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>