objectbox-rxjava: Add README.

Author: greenrobot-team

objectbox-rxjava/README.md

@@ -0,0 +1,31 @@
+RxJava 2 APIs for ObjectBox
+===========================
+While ObjectBox has [data observers and reactive extensions](https://docs.objectbox.io/data-observers-and-rx) built-in,
+this project adds RxJava 2 support.  
+
+For general object changes, you can use `RxBoxStore` to create an `Observable`.
+
+`RxQuery` allows you to interact with ObjectBox `Query` objects using:
+ * Flowable
+ * Observable
+ * Single
+
+For example to get query results and subscribe to future updates (Object changes will automatically emmit new data):
+
+```java
+Query query = box.query().build();
+RxQuery.observable(query).subscribe(this);
+```
+    
+Adding the library to your project
+-----------------
+Grab via Gradle:
+```gradle
+compile 'io.objectbox:objectbox-rxjava:objectboxVersion'
+```
+
+Links
+-----
+[Data Observers and Rx Documentation](https://docs.objectbox.io/data-observers-and-rx)
+
+[Note App example](https://github.com/objectbox/objectbox-examples/blob/master/objectbox-example/src/main/java/io/objectbox/example/ReactiveNoteActivity.java)