contrib/wix: add COPYING.rtf
Moving the file from the misc directory of the thg-winbuild repo, which
uses mercurial's contrib/wix.
This file is displayed by the install UI sequence, so it belongs to the
WiX sources quite closely.
It happens to be installed by the msi installer, too.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://mercurial.selenic.com/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.