Make githooks.checkjira.matchlog option be multi-valued

This allows one to specify more than one place in the commit message
where JIRA keys are looked for.

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Improve README

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>