Add snippet for reloading on read replicas

[steve-chavez]

Related to PostgREST/postgrest#2781.

I thought of adding the following to http://postgrest.org/en/stable/references/schema_cache.html#automatic-schema-cache-reloading

  IF pg_is_in_recovery()
  THEN
    PERFORM pg_terminate_backend(pid) FROM pg_stat_activity 
    WHERE application_name ilike '%postgrest%';
  ELSE
	  NOTIFY pgrst, 'reload schema';
  END IF;

But terminating the connection is not graceful for a cache reload as requests will fail once.

There might be a way forward with PostgREST/postgrest#1766 (comment), so I'll hold this snippet until that's settled.