Add requirement for APIImpact flag

Adds a requirement for an APIImpact flag in
the commit message for a proposed spec if it proposes
changes to the Zaqar REST API. This will make it
much easier for people such as the API WG who want to
review API changes across OpenStack to find and
review proposed API changes.

Change-Id: Ib5f348764329864ea3f312a8c19c4cd322631909

template.rst

@@ -34,6 +34,15 @@ propose to solve this problem?
 If this is one part of a larger effort make it clear where this piece ends. In
 other words, what's the scope of this effort?
 
+If your specification proposes any changes to the Zaqar REST API such
+as changing parameters which can be returned or accepted, or even
+the semantics of what happens when a client calls into the API, then
+you should add the APIImpact flag to the commit message. Specifications with
+the APIImpact flag can be found with the following query:
+
+https://review.openstack.org/#/q/status:open+project:openstack/zaqar-specs+message:apiimpact,n,z
+
+
 Alternatives
 ------------