Allow introductory text in commit log and ignore it when generating ChangeLog

Allow introductory text in commit log and ignore it when generating ChangeLog

	* gen-changelog.py (process_commit): Everything that comes between
	the subject line of the commit and the first \t* sequence is
	considered to be introductory text.  Ignore it when generating the
	ChangeLog entry.
	* COMMIT-LOG-GUIDELINES: Update the commit log guidelines to
	reflect the fact that we can now have introductory text in the
	commit log.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>