[docs] Update README for 0.4.0 release.

Closes #409

README.md

@@ -32,9 +32,9 @@ Getting Started
 1. Download the latest release of YCSB:
 
     ```sh
-    curl -O --location https://github.com/brianfrankcooper/YCSB/releases/download/0.3.1/ycsb-0.3.1.tar.gz
-    tar xfvz ycsb-0.3.1.tar.gz
-    cd ycsb-0.3.1
+    curl -O --location https://github.com/brianfrankcooper/YCSB/releases/download/0.4.0/ycsb-0.4.0.tar.gz
+    tar xfvz ycsb-0.4.0.tar.gz
+    cd ycsb-0.4.0
     ```
     
 2. Set up a database to benchmark. There is a README file under each binding