Add minSdkVersion note to build.gradle.

library/build.gradle

@@ -19,6 +19,11 @@ android {
     buildToolsVersion "21.1.2"
     buildToolsVersion "21.1.2"
 
 
     defaultConfig {
     defaultConfig {
+        // Important: ExoPlayerLib specifies a minSdkVersion of 9 because
+        // various components provided by the library may be of use on older
+        // devices. However, please note that the core video playback
+        // functionality provided by the library requires API level 16 or
+        // greater.
         minSdkVersion 9
         minSdkVersion 9
         targetSdkVersion 21
         targetSdkVersion 21
     }
     }