Add tutorial on how to generate APPLgrids with MG5

Created by: voisey

This adds the instructions from this branch https://github.com/NNPDF/external/tree/README to the sphinx docs. I guess that once this is merged that branch can be deleted. I followed Emanuele's instructions and didn't have any problems in producing some grids. I've edited the original instructions very slightly and I've added a few extra details. I have also edited the format of Rabah's MCFM instructions slightly to fit with the new subsection on MG5.

One thing I think could be useful would be to have instructions on what to do once one has generated APPLgrids. E.g. how should one test that they work as expected, and what should one do with the grids if they are implementing a new experiment. What do you think? If you agree that this would be useful, should we address this here or shall we merge this and I can open an issue?