Update REVIEWING.rst for check-requirements job

Recent review Id8e5d55e34ec3b3d555b5482e13f99c2561e871e removed
check-requirements job without removing the project from
projects.txt in requirements repo. It caused a voting gate to
fail all the reviews submitted.

Updated REVIEWING.rst to verify this before approving
removal of check-requirements job.

Change-Id: I7b5e7665411117db1f524cd644f1a9c645e6cbcb

REVIEWING.rst

@@ -66,3 +66,14 @@ Here are some points to look at:
 
 * Proposal jobs should not run arbitrary scripts from other
   repositories or install untrusted packages.
+
+Removing check-requirements job
+===============================
+
+* The http://git.openstack.org/cgit/openstack/requirements repository holds
+  list of projects that have check-requirements job in <root>/projects.txt.
+
+* Whenever a review is submitted for removing check-requirements job,
+  the reviewer should verify that the project has been removed from
+  projects.txt already or that this project-config change depends
+  on the review to remove it.