Add information about inline reviews

Add a paragraph explaining how to comment on specific lines of a review.

Also, add link to Gerrit review documentation.

The Paragraph was copied from https://review.openstack.org/107360 and markup
adjusted.

Change-Id: Id2e848c0ec20d87b504610b2aed492f9c22d9e35
Co-Authored-By: Pavel Sedlák <psedlak@redhat.com>
Merge "Add information about inline reviews"