Add suggested acknowledgements to the README

@boris.goncharov suggested we add suggested acknowledgement with some advise. This sounds like a good idea and one we should make sure is done carefully to get all relevant citations.

I would suggest we say something along the lines of

If you use bilby, we would appreciate that you cite XXX. We also recommend that you provide a citation for the specific sampler(s) that you make use of (see list YYY below) and if you used the gw-specific module additionally cite packages ZZZ