Add a link to the docs from the README

Newcomers often struggle to find the docs for Oslo libraries. Let's
help them out a little.

Change-Id: I68cb214ad117ff1c45e83df302f37dba485147f6
Add log_handler to oslo.messaging

Move log_handler from oslo-incubator to oslo.messaging and include
unit tests. This patch also adds an additional test case from the
original version in oslo-incubator. Originally, the test case only
checked to make sure the notification was not sent. Now a positive test
case exists to test both paths.

bp graduate-notifier-log-handler
Co-Authored By: Ben Nemec <bnemec@redhat.com>

Change-Id: I9ea051ec7e3614579341a31d8dc1c13d9514b8a9
Merge "Add log_handler to oslo.messaging"
Merge "Add a link to the docs from the README"