add last_unit_added and last_unit_removed to repo docs
Merge pull request #1220 from pulp/barnabycourt-doc-unit-add-remove-timestamps

add last_unit_added and last_unit_removed to repo docs
Automatic commit of package [pulp] release [2.5.0-0.8.beta].
Automatic commit of package [pulp-nodes] release [2.5.0-0.8.beta].
Merge remote-tracking branch 'origin/2.5-testing' into 2.5-dev