Update Cronet extension readme on how to enable Java 8 features.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=180660349

extensions/cronet/README.md

@@ -19,10 +19,20 @@ and enable the extension:
 1. Copy the three jar files into the `libs` directory of this extension
 1. Copy the content of the downloaded `libs` directory into the `jniLibs`
    directory of this extension
-
-* In your `settings.gradle` file, add
-  `gradle.ext.exoplayerIncludeCronetExtension = true` before the line that
-  applies `core_settings.gradle`.
+1. In your `settings.gradle` file, add
+   `gradle.ext.exoplayerIncludeCronetExtension = true` before the line that
+   applies `core_settings.gradle`.
+1. In all `build.gradle` files where this extension is linked as a dependency,
+   add
+   ```
+   android {
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+   }
+   ```
+   to enable Java 8 features required by the Cronet library.
 
 [top level README]: https://github.com/google/ExoPlayer/blob/release-v2/README.md
 [here]: https://console.cloud.google.com/storage/browser/chromium-cronet/android