Update README.md

Small fix to avoid error if the timescaledb extension is already installed

Signed-off-by: Ramon Guiu <[email protected]>

README.md

@@ -40,7 +40,7 @@ You create a regular table and then convert it into a hypertable. A hypertable a
 
 ```sql
 -- Create timescaledb extension
-CREATE EXTENSION timescaledb;
+CREATE EXTENSION IF NOT EXISTS timescaledb;
 
 -- Create a regular SQL table
 CREATE TABLE conditions (