GDP: three additional instructions for helpers

[Graham Percival]

As an aside, I'm starting every email with "GDP: " in the subject line 
so that people can easily filter them.  If you care about GDP, make a 
new mailbox for these emails; if you don't care about it, then send all 
these emails straight to the trash.  :)


I've added some more instructions to the readme.txt, but you probably 
wouldn't read that file again normally, so I'm sending them here.

GDP: if you see a @refcommands, move those items into the top of the
@commonprop section and delete the @refcommands.
 (Valentin: your offer with regards to this is appreciated but not 
necessary, there's a better way to do it)

GDP: if you see any TODO items, send them to Graham and delete
them from the file (I'll add them to my master TODO list for that
section).  We will resolve all these issues before declaring a doc
section finished.

GDP: treat any questions in comments like TODO items.

Cheers,
- Graham