Add xcb-dpms to requirements documentation

[williballenthin]

Building on Ubuntu 13.04, requires xcb-dpms, which is provided by libxcb-dpms0-dev.

 » make
fatal: No names found, cannot describe anything.
Package xcb-dpms was not found in the pkg-config search path.
Perhaps you should add the directory containing `xcb-dpms.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xcb-dpms' found

[shiver]

Thank you for reporting the issue.

While I do agree that the documentation does not detail all the requirements, I would encourage you to report this issue upstream (http://cr.i3wm.org) as the fork doesn't actually introduce any new dependencies.

I have however, made the changes for this and #2.