Add note about metadata.product version .qualifier (#2227)

Author: briandealwis

gcp-repo/metadata.product

@@ -3,6 +3,11 @@
 <!--
   This product exists to (1) bring in any required features, and
   (2) add any additional metadata for the CT4E repository.
+
+  The product version should have a .qualifier *except* for releases.
+  This version is used for creating our CT4E category (see the accompanying
+  .p2.inf).  p2 generates unique qualifiers for categories, so it would be
+  best for our non-release builds to have unique versions too.
 -->
 <product uid="com.google.cloud.tools.eclipse.dist" version="1.3.0.qualifier" useFeatures="true" includeLaunchers="false">