[bug-gv] add State Saving Tutorial

[積丹尼 Dan Jacobson]

Idea: have some way for the user, after getting everything just right
via the mouse, keyboard, etc. to be able to extract the current state,
so next time he can reproduce the state next time, via command line
switches, etc.

OK I found "State" at the top and pressed Save.

Well it should print a message saying where it saved it. (Even though I found 
it, ~/.gv .)

NOPE, SORRY it is not clear how to do this from the manual.

The manual talks about --ad etc. Sorry. Please add a state saving
tutorial section. E.g.,

State Saving Tutorial
*********************

Every month user Samantha views her telephone bill (PDF) with gv, and it
takes quite a while before she get it zoomed, centered, and full-screened
just right.

She would like to extract the state, so next month all she needs to do is
have a bash alias
phone_bill_view='gv -a 111 -b 222 -c 333 bill.pdf'

OK, let's get started.

$ gv bill.pdf
Now press State | Presentation Mode
Now zoom till just right
Now [Do something that saves the state][Cannot reach the state button
here in Presentation mode, by the way.]

Then next month, use alias phone_bill_view='....'.