Missing dependancies from documentation

[Pouya-moh]

Hi,

A number of dependencies are missing from our dependencies:

1. Eigen 3 (libeigen3-dev) needed, well, by everyone! I think we implicitly assumed everyone has it...
2. libyaml-cpp-dev, that IMHO, is mother of all evils and is needed by xbot. We have this in our citk projects so adding it to the distribution should be straightforward.
3. libffi-dev

The dependency documentation is hosted outside of this repo which means we cannot directly fix these issues. In a separate issue I will propose to move them to this repo.