[doc] logging: explain how to enable it in junit tests

docs/android_logging.md

Index: docs/android_logging.md
diff --git a/docs/android_logging.md b/docs/android_logging.md
index c8404e3ba461944780747ed9307d0920ea74ab57..382d2f19048bdcb6ec82f67f770e2504a74c83dd 100644
--- a/docs/android_logging.md
+++ b/docs/android_logging.md
@@ -206,3 +206,23 @@ further.
 
 For more, see the [related page on developer.android.com]
 (http://developer.android.com/tools/debugging/debugging-log.html#filteringOutput)
+
+## Logs in JUnit tests
+
+We use [robolectric](http://robolectric.org/) to run our JUnit tests. It
+replaces some of the Android framework classes with "Shadow" classes
+to ensure that we can run our code in a regular JVM. `android.util.Log` is one
+of those replaced classes, and by default calling `Log` methods doesn't print
+anything.
+
+That default is not changed in the normal configuration, but if you need to
+enable logging locally or for a specific test, just add those few lines to your
+test:
+
+```java
+@Before
+public void setUp() {
+  ShadowLog.stream = System.out;
+  //you other setup here
+}
+```