parse-headers.pl: add documentation for this script

Provide a man page for parse-headers.pl, describing
how to use it.

The documentation on ReST format was generated via pod2rst:
	http://search.cpan.org/~dowens/Pod-POM-View-Restructured-0.02/bin/pod2rst



Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>