Add documentation on how to export the right sort of files from CST

[bhazelton]

We now have a read_cst method that works if the beams are exported properly from CST. The consensus is that rather than trying to read all sorts of CST beams, we should document how to export the right sort of beams from CST (e.g. with screen shots).

[bhazelton]

@anguta @zacharymartinot Do you have a document that addresses this issue? I heard that you were working on such a thing...

[bhazelton]

@jaguirre do these docs exist somewhere?

[bhazelton]

@jaguirre @zacharymartinot @anguta Does documentation of this sort exist? I have heard several times that some superposition of you three were working on it.