[Release] Update top level README.md file to point to 0.9.0 release.

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/0.8.0/ycsb-0.8.0.tar.gz
-    tar xfvz ycsb-0.8.0.tar.gz
-    cd ycsb-0.8.0
+    curl -O --location https://github.com/brianfrankcooper/YCSB/releases/download/0.9.0/ycsb-0.9.0.tar.gz
+    tar xfvz ycsb-0.9.0.tar.gz
+    cd ycsb-0.9.0
     ```
     
 2. Set up a database to benchmark. There is a README file under each binding