Instruction on using the models in Gazebo should be updated to consider also the case in which the models are used from the build directory

[traversaro]

In https://github.com/robotology/icub-models#use-the-models-with-gazebo, we should also consider the case of models used from the build, as that case is documented as supported in the superbuild itself.

[diegoferigo]

I'm confused, would it be different than https://github.com/robotology/icub-models#from-the-source-repo?

Edit: ow, now I got the right anchor to the README. Yes it is missing.