Add note about default pg pool size (#286)

Author: benjie

src/pages/postgraphile/usage-library.md

@@ -301,8 +301,11 @@ The `postgraphile` middleware factory function takes three arguments, all of
 which are optional. The below options are valid for
 <tt>postgraphile@<!-- LIBRARY_VERSION_BEGIN -->4.12.3<!-- LIBRARY_VERSION_END --></tt>.
 
-- **`pgConfig`**: An object or string that will be passed to the [`pg`][]
-  library and used to connect to a PostgreSQL backend, OR a pg.Pool to use.
+- **`pgConfig`**: Specifies the PostgreSQL database you wish to connect to.
+  You may pass a PostgreSQL connection string, a configuration object to pass
+  to the [`pg.Pool`][] constructor, or a [`pg.Pool`][] instance. Note: `pg.Pool`
+  has a default pool size of 10, to increase this you use the configuration
+  object/pg.Pool instance approach and pass a different value as `max`.
 - **`schemaName`**: A string, or array of strings, which specifies the
   PostgreSQL schema(s) you to expose via PostGraphile; defaults to 'public'
 - **`options`**: An object containing other miscellaneous options. Options
@@ -739,4 +742,5 @@ You can find
 [express]: https://www.npmjs.com/express
 [graphql/express-graphql#82]: https://github.com/graphql/express-graphql/pull/82
 [`pg`]: https://www.npmjs.com/pg
+[`pg.Pool`]: https://node-postgres.com/api/pool
 [morgan]: https://www.npmjs.com/morgan