[release] Update README for the release of YCSB 0.15.0

closes #1204

README.md

@@ -32,9 +32,9 @@ Getting Started
 1. Download the [latest release of YCSB](https://github.com/brianfrankcooper/YCSB/releases/latest):
 
     ```sh
-    curl -O --location https://github.com/brianfrankcooper/YCSB/releases/download/0.14.0/ycsb-0.14.0.tar.gz
-    tar xfvz ycsb-0.14.0.tar.gz
-    cd ycsb-0.14.0
+    curl -O --location https://github.com/brianfrankcooper/YCSB/releases/download/0.15.0/ycsb-0.15.0.tar.gz
+    tar xfvz ycsb-0.15.0.tar.gz
+    cd ycsb-0.15.0
     ```
     
 2. Set up a database to benchmark. There is a README file under each binding