Fix README.md

Author: nadbp

README.md

@@ -59,6 +59,17 @@ We test this foreign data wrapper on Ubuntu Server 18.04 using PostgreSQL-11 tog
 
 # Usage
 
+Before using this foreign data wrapper, you need to add it to ```shared_preload_libraries``` in your ```postgresql.conf``` 
+
+    shared_preload_libraries = 'kv_fdw'    # (change requires restart)
+
+and restart Postgres:
+
+
+```sh
+    sudo service postgresql restart  
+```
+
 This extension does not have any parameter. After creating the extension and corresponding server, you can use RocksDB as a foreign storage engine for your PostgreSQL.
 
 A simple example is as follows.