Docs: Update Readme (#317)

* Docs: Update javadoc link, add note about merged async driver

* Async example link

Author: Simran-B

docs/Drivers/Java/README.md

@@ -2,6 +2,12 @@
 
 The official ArangoDB Java Driver.
 
+It can be used synchronously as well as asynchronously. The formerly separate async
+driver with the same API as the synchronous driver, except that it returned a
+`CompletableFuture<T>` instead of the result `T` directly, was merged into this
+driver in version 6.2.0. See
+[async examples](https://github.com/arangodb/arangodb-java-driver/tree/master/src/test/java/com/arangodb/async/example){:target="_blank"}.
+
 - [Getting Started](GettingStarted/README.md)
 - [Reference](Reference/README.md)
 
@@ -10,5 +16,5 @@ The official ArangoDB Java Driver.
 - [ChangeLog](https://raw.githubusercontent.com/arangodb/arangodb-java-driver/master/ChangeLog.md)
 - [Examples](https://github.com/arangodb/arangodb-java-driver/tree/master/src/test/java/com/arangodb/example)
 - [Tutorial](https://www.arangodb.com/tutorials/tutorial-sync-java-driver/)
-- [JavaDoc](http://arangodb.github.io/arangodb-java-driver/javadoc-4_3/index.html)
+- [JavaDoc](http://arangodb.github.io/arangodb-java-driver/javadoc-6_3/index.html)
 - [JavaDoc VelocyPack](http://arangodb.github.io/java-velocypack/javadoc-1_0/index.html)