Create and add 'Cheat Sheet' to extend cf-python one

[sadielbartholomew]

It would be nice to have a Cheat Sheet document to summarise important function calls and outputs and in general the utility of cf-plot.

We currently have a fantastic one for cf-python, and that has the benefit of working both in online format and as a printed document. I suggest we design our Cheat sheet' so that it extends the cf-python one to showcase the plotting capability with cf-plot after set up with cf-python as per the cf-python Cheat Sheet. If we can create an identical page format, then the cf-plot cheat sheet can also be printed out and sit alongside the cf-python one to form an overall cf* NCAS Data Tools Cheat Sheet that is twice the size but the individual documents can also work standalone.