Add Wiki page / documentation / instructions for wcd_pth_migration

[casedefault]

I am going to look through the code myself to see if I can work it out, but from what I understand there's a decent tool in here that almost nobody has used because they don't know how.

@karamanolev you wrote this, I think?

I know you're busy and a detailed wiki might not be possible but I for one would greatly appreciate even just a couple of pointers to get started using this. So far I don't even know whether it's designed to be run on my old WCD instance of WM2 and pointed at the new RED install, or vice versa.