[PATCH] adjust and clarify build instructions

[Charles Oliver Nutter]

John Rose wrote:
> For this one, too, may I have a reviewer?  Thanks.  -- John
> 
> P.S.  Reviews can include suggestions for improvement!

Looks good to me, and now includes the two key commands I had to hunt 
for: qpop and qpush.

No suggestions for improvement at the moment, because I know exactly 
today's worth of fiddling about hg.

- Charlie