Add tags for compiler properties (#299)

1. tags for compiler properties
2. gradle comment

Author: lesv

appengine/helloworld-new-plugins/build.gradle

@@ -11,7 +11,7 @@ buildscript {
 repositories {
   maven { 
     url 'https://maven-central.storage.googleapis.com'            // Google's mirror of Maven Central
-//     url 'https://oss.sonatype.org/content/repositories/snapshots' // SNAPSHOT Reposiotry
+//     url 'https://oss.sonatype.org/content/repositories/snapshots' // SNAPSHOT Reposiotry (if needed)
   }
   jcenter()
   mavenCentral()

appengine/helloworld-new-plugins/pom.xml

@@ -21,17 +21,20 @@ Copyright 2015 Google Inc. All Rights Reserved.
   <groupId>com.example.appengine</groupId>
   <artifactId>appengine-helloworld-newplugins</artifactId>
 
-    <properties>
-        <maven.compiler.target>1.7</maven.compiler.target>
-        <maven.compiler.source>1.7</maven.compiler.source>
-    </properties>
+<!-- [START compiler] -->
+  <properties>  <!-- App Engine Standard currently requires Java 7 -->
+    <maven.compiler.target>1.7</maven.compiler.target>
+    <maven.compiler.source>1.7</maven.compiler.source>
+  </properties>
+<!-- [END compiler] -->
 
   <parent>
     <groupId>com.google.cloud</groupId>
     <artifactId>doc-samples</artifactId>
     <version>1.0.0</version>
     <relativePath>../..</relativePath>
   </parent>
+
   <dependencies>
     <dependency>
       <groupId>javax.servlet</groupId>
@@ -40,6 +43,7 @@ Copyright 2015 Google Inc. All Rights Reserved.
       <scope>provided</scope>
     </dependency>
   </dependencies>
+
   <build>
     <!-- for hot reload of the web application -->
     <outputDirectory>${project.build.directory}/${project.build.finalName}/WEB-INF/classes</outputDirectory>