Add note about requiring an <instrumentation> tag in a test app's And…

…roidManifest.xml

RELNOTES: None.
PiperOrigin-RevId: 208725953

site/docs/android-instrumentation-test.md

@@ -170,6 +170,36 @@ The main attributes of the rule `android_instrumentation_test` are:
   run the tests. See the [section on choosing an Android
   device](#choosing-an-android_device) for more information.
 
+The test app's `AndroidManifest.xml` must include
+[an `<instrumentation>` tag](https://developer.android.com/studio/test/#configure_instrumentation_manifest_settings).
+This tag must specify the attributes for the **package of the target app** and
+the **fully qualified class name of the instrumentation test runner**,
+`android.support.test.runner.AndroidJUnitRunner`.
+
+Here is an example `AndroidTestManifest.xml` for the test app:
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+          xmlns:tools="http://schemas.android.com/tools"
+          package="com.example.android.app.test"
+    android:versionCode="1"
+    android:versionName="1.0">
+
+    <instrumentation
+        android:name="android.support.test.runner.AndroidJUnitRunner"
+        android:targetPackage="com.example.android.app" />
+
+    <uses-sdk
+        android:minSdkVersion="16"
+        android:targetSdkVersion="27" />
+
+    <application >
+       <!-- ... -->
+    </application>
+</manifest>
+```
+
 ## `WORKSPACE` dependencies
 
 In order to use this rule, your project needs to depend on these external