Add SQL for grant connect (prometheus-community#303)

To resolve the error below, it was necessary to add a new grant to the database postgres.

Error: 'pq: permission denied for database \ "postgres \"'

Author: focabr

README.md

@@ -208,6 +208,7 @@ ALTER USER postgres_exporter SET SEARCH_PATH TO postgres_exporter,pg_catalog;
 -- GRANT postgres_exporter TO <MASTER_USER>;
 CREATE SCHEMA IF NOT EXISTS postgres_exporter;
 GRANT USAGE ON SCHEMA postgres_exporter TO postgres_exporter;
+GRANT CONNECT ON DATABASE postgres TO postgres_exporter;
 
 CREATE OR REPLACE FUNCTION get_pg_stat_activity() RETURNS SETOF pg_stat_activity AS
 $$ SELECT * FROM pg_catalog.pg_stat_activity; $$