Refine install instruction

Author: kulaginm

README.md

@@ -96,12 +96,13 @@ shared_preload_libraries = 'pg_pathman'
 It is essential to restart the PostgreSQL instance. After that, execute the following query in psql:
 ```plpgsql
 CREATE SCHEMA pathman;
+GRANT USAGE ON SCHEMA pathman TO PUBLIC;
 CREATE EXTENSION pg_pathman WITH SCHEMA pathman;
 ```
 
 Done! Now it's time to setup your partitioning schemes.
 
-> **Security notice**: Do not install pg_pathman in public schemas! Specify the schema explicitly using the WITH SCHEMA clause. Installing an extension in a scheme that is accessible to malicious users can lead to a privilege escalation attack.
+> **Security notice**: Do not install pg_pathman in public schemas! Specify the schema explicitly using the WITH SCHEMA clause. Installing an extension into a schema in which attackers can create objects can lead to an elevation of privilege attack.
 
 > **Windows-specific**: pg_pathman imports several symbols (e.g. None_Receiver, InvalidObjectAddress) from PostgreSQL, which is fine by itself, but requires that those symbols are marked as `PGDLLIMPORT`. Unfortunately, some of them are not exported from vanilla PostgreSQL, which means that you have to either use Postgres Pro Standard/Enterprise (which includes all necessary patches), or patch and build your own distribution of PostgreSQL.