Give more information on how to use beamformers (module docstring or example notebook)

[mberz]

If this becomes an example, this issue should be moved to the gallery, otherwise I'd suggest to work on this not earlier than v.1.1.0

[f-brinkmann]

We've updated the docs and I agree that adding a notebook for this is not important for 1.0. Hence removing this as a subissure in #216 and putting in on hold