Document 'rosdep-needs-second-approval' label in REVIEW_GUIDELINES.md (#44939)

Signed-off-by: Christophe Bedard <[email protected]>
Co-authored-by: Tully Foote <[email protected]>

Author: christophebedard

REVIEW_GUIDELINES.md

@@ -69,6 +69,8 @@ There are a few different types of pull requests that are opened against this re
     * New rosdep keys are typically only accepted for supported Ubuntu distros which have ROS releases; the ROS support timeline generally follows the Ubuntu LTS support timeline.
     * If there is no good alternative for a pip package to add to rosdep keys, use `python3-PACKAGENAME-pip` to name the key.
     * If a key conforms to all standards above, then it can be approved. Once the pull request has two approvals, it will be merged.
+        * To help make pull requests that are only waiting for a second approval & merge more visible, the `rosdep-needs-second-approval` label can be used.
+        * Also please request review explicitly from the other person on the rosdistro review rotation.
 
 1.  A revert of an existing package.  This can happen if the package in question can’t be built on the farm for some reason, or the maintainer doesn’t want to maintain it, or for various other reasons.  An example of this kind of PR is [26427](https://github.com/ros/rosdistro/pull/26427).  If a package was previously merged into rosdistro, but has never been synced to main, these will be merged right away (no downstream users could have installed them).  If a package *has* been synced to main, it is still safe to remove it.  However, the submitter should be aware that the package will disappear from ROS completely; no old versions will be kept around for users to install.  The reviewer will ensure that the maintainer is aware of that limitation.  If the submitter is OK with that situation, then these will be merged.  It can be determined if a package has been synced to main by visiting either [ROS 1 Status](http://repositories.ros.org/status_page) or [ROS 2 Status](http://repo.ros2.org/status_page/), and clicking on the distribution in question.  For instance, to look up Melodic, click on [http://repositories.ros.org/status_page/ros_melodic_default.html](http://repositories.ros.org/status_page/ros_melodic_default.html).  The package can be searched for on the top bar.  The three boxes to the right of the package name determine its status in the "building", "testing", and "main" repositories.  If a package is red in "main", then it has never been synced to main.