SUPPORT.md: Syntax: Provide a title rather than a spurious empty section

SUPPORT.md: Syntax: Provide a title rather than a spurious empty section

This commits (more or less) this file to be processed with pandoc,
rather than other markdown processors.  There is, unfortunately, no
widely-accepted way to declare a title for the document.

I tested feeding the document to markdown(1) on Debian jessie and it
reproduced the % line as if it were simple text.  I guess many other
markdown processors will do something similarly tolerable.  My
internet searches did not discover a markdown processor that used
lines starting with % for something else.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Lars Kurth <lars.kurth@citrix.com>