explain the default db for postgres

postgres/README.md

@@ -10,7 +10,8 @@ PostgreSQL implements the majority of the SQL:2011 standard, is ACID-compliant a
 ## start a postgres instance
     docker run --name some-postgres -d postgres
 
-This image includes `EXPOSE 5432` (the postgres port), so standard container linking will make it automatically available to the linked containers (as the following examples illustrate).
+This image includes `EXPOSE 5432` (the postgres port), so standard container linking will make it automatically available to the linked containers. The default `postgres` user and database are created in the entrypoint with `initdb`.
+> The postgres database is a default database meant for use by users, utilities and third party applications. [postgresql.org/docs](http://www.postgresql.org/docs/9.3/interactive/app-initdb.html)
 
 ## connect to it from an application
     docker run --name some-app --link some-postgres:postgres -d application-that-uses-postgres