README

manpage build: fail early when xmlto is not available

When we try to build manpages with xmlto and sed, but xmlto is
missing fail at the xmlto stage.  Otherwise, one may run `cd doc;
make' and miss the warnings like:

  xmlto: not found
  sed: hg.1: No such file or directory

and end up with empty files installed as manpages.

Basic install:

 $ make            # see install targets
 $ make install    # do a system-wide install
 $ hg debuginstall # sanity-check setup
 $ hg              # see help

See http://www.selenic.com/mercurial/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.