Add some SQL in README.md

Getting the error when adding non-root postgres user for exporter: ``` postgres=# CREATE OR REPLACE FUNCTION get_pg_stat_statements() RETURNS SETOF pg_stat_statements AS postgres-# $$ SELECT * FROM public.pg_stat_statements; $$ postgres-# LANGUAGE sql postgres-# VOLATILE postgres-# SECURITY DEFINER; ERROR: type "pg_stat_statements" does not exist ``` Add following sql command for enable pg_stat_statements: ``` CREATE EXTENSION IF NOT EXISTS pg_stat_statements; ``` Signed-off-by: Xela Tirdan <xelatirdan@gmail.com>

Author: xelatirdan

README.md

@@ -267,6 +267,7 @@ AS
 
 GRANT SELECT ON postgres_exporter.pg_stat_replication TO postgres_exporter;
 
+CREATE EXTENSION IF NOT EXISTS pg_stat_statements;
 CREATE OR REPLACE FUNCTION get_pg_stat_statements() RETURNS SETOF pg_stat_statements AS
 $$ SELECT * FROM public.pg_stat_statements; $$
 LANGUAGE sql