Update appengine-web.xml (#230)

* Update appengine-web.xml

Add region tags for documentation.

* Update appengine-web.xml

Author: carieshmarie

appengine/mailjet/src/main/webapp/WEB-INF/appengine-web.xml

@@ -17,8 +17,10 @@
   <application>YOUR-PROJECT-ID</application>
   <version>YOUR-VERSION-ID</version>
   <threadsafe>true</threadsafe>
+// [START env_variables]
   <env-variables>
     <env-var name="MAILJET_API_KEY" value="YOUR-MAILJET-API-KEY" />
     <env-var name="MAILJET_SECRET_KEY" value="YOUR-MAILJET-SECRET-KEY" />
   </env-variables>
+// [END env_variables]
 </appengine-web-app>