expanded instructions for new user of wfdiff

[carltape]

It would be great to have some basic startup instructions (README?) for new users. Notably:

1. create two separate specfem folders, one for NGLL=5, the other for NGLL=7
2. to set NGLL=7, you need to modify the constants file, then recompile
3. for NGLL=7, you'll need a lower time step (about half the size); whatever factor you reduce the time step by, you need to extend NSTEP by, in order to have the same length of seismogram time for comparison
   We can assume that a new user has run specfem for NGLL=5 (default) -- so here we'd be explaining how to set up the run for NGLL=7.