README.md: Small improvements in text and layout

* Fixup to have numbered lists display properly
* Add links to: Franca IDL, Vagrant, VirtualBox
* A few other minor changes

Signed-off-by: Gianpaolo Macario <gianpaolo_macario@mentor.com>
Merge pull request #3 from gmacario/fix-readme

README.md: Small improvements in text and layout