Improve skeleton readme
Adds installation instructions to the README generated by the cubicweb-ctl newcube command.
The README is in reST format and explains to new users how to add the cube as a dependency and how to use it as an instance. It also links to the documentation for more information (with the version of CubicWeb used to generate the cube).
Here is an example of a README rendered in PyCharm for a new cube named templatetest:
