Add README for Android library.

Author: greenrobot-team

eventbus-android/README.md

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+# EventBus for Android
+
+Despite its name this module is actually published as `org.greenrobot:eventbus` as an Android library (AAR).
+
+It has a dependency on the Java-only artifact `org.greenrobot:eventbus-java` (JAR) previously available under the `eventbus` name.
+
+Provides an `AndroidComponents` implementation to the Java library if it detects `AndroidComponentsImpl` on the classpath via reflection.