QF210922-1 -- Fix missing tag (#583)

Author: ibsoln

modules/ROOT/pages/_partials/common-releasenotes.adoc

@@ -187,7 +187,20 @@ More::
 // end::new-300-auto-purge[]
 // end::new-300-auto-purge-full[]
 
-// end::feature1-300[]
+// tag::new-300-environment-vars[]
+// tag::new-300-environment-vars-full[]
+// tag::new-300-environment-vars-summ[]
+Sync Gateway configuration is extended to allow the use of defined _environment variables_ as substitution values inside the configuration file.
+This allows users to determine, pick-up and substitute appropriate values during Sync Gateway start-up.
+
+// end::new-300-environment-vars-summ[]
+The use of environment variables within the configuration file increases the flexibility of the configuration process.
+It makes the switching of the runtime behavior -- for example during development, experimentation or testing -- much easier.
+
+// end::new-300-environment-vars-full[]
+More::
+  xref:configuration-environment-variables.adoc[Configuration Environment Variables]
+// end::new-300-environment-vars[]
 
 // tag::feature1-300[]