git attributes: Don't merge CNAME
git attributes: Don't merge CNAME

The CNAME file is used by github when serving the main web page. We
want to avoid using it on gh-pages branches that we use for testing.

Signed-off-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>

tianocore/tianocore.github.io,tianocore/tianocore.github.io,jljusten/tianocore,jljusten/tianocore