checkcommits: Improve docs.
checkcommits: Improve docs.

Simplify Travis example and add a couple of new sections
showing basic usage and an example of using various options.

Signed-off-by: James O. D. Hunt <348fbaadfbada55ea105602bf5609e23ac1c4898@intel.com>

bwarden/tests,GabyCT/tests,devimc/tests,bwarden/tests,GabyCT/tests,sboeuf/tests,clearcontainers/tests,chavafg/tests,devimc/tests,MarioCarrilloA/tests,clearcontainers/tests,chavafg/tests,MarioCarrilloA/tests,sboeuf/tests