Add a comment about keeping gradle version in sync with manifest file.

Test: None
Change-Id: I161fc2364016ac9ecae4821ad79676c8c752fb93
diff --git a/build.gradle b/build.gradle
index 76c211a..0ce76e2 100644
--- a/build.gradle
+++ b/build.gradle
@@ -14,6 +14,7 @@
         maven { url "../../prebuilts/maven_repo/android" }
     }
     dependencies {
+        // Keep gradle plugin version in sync with ub_supportlib-master manifest.
         classpath 'com.android.tools.build:gradle:2.2.0'
     }
 }