Convert the README.rst minifesto back to the "real" README

[qstokkink]

Currently, the README.rst is a small manifesto to describe the different approach of Tribler Experimental as opposed to "real" Tribler.

If the Tribler Experimental is to be a real standalone product, it should also have a "real" README again. Perhaps simply a revert to the previous state.

[qstokkink]

The current "main" Tribler README is not compatible with TriblerExperimental. For it to make sense in this repository, half of the information should be removed (and then added again later if TriblerExperimental graduates into Tribler).

In other words, it's a better idea to leave this until later.