Explain database superuser

README.md

@@ -72,12 +72,16 @@ The recommended way to run this container is by linking it to a PostgreSQL datab
 You're free to pick (or build) any PostgreSQL container, as long as it exposes
 its database port (5432) to the outside.
 
-Example:
+Example with nornagon/postgres:
 
 ```bash
 $ docker run -d --name=tinydatabase nornagon/postgres:latest
 ```
 
+> The image nornagon/postgres exposes a database superuser that this image uses
+to automatically create its user and database,
+so you don't have to setup your database credentials here.
+
 Use the following database options when running the container:
 
 ```
@@ -91,12 +95,15 @@ mysql container instead.
 You're free to pick (or build) any MySQL container, as long as it exposes
 its database port (3306) to the outside.
 
-Example:
+Example with sameersbn/mysql:
 
 ```bash
-$ docker run -d --name=tinydatabase sameersbn/mysql:latest
+$ docker run -d --name=tinydatabase -e DB_USER=ttrss -e DB_PASS=ttrss -e DB_NAME=ttrss sameersbn/mysql:latest
 ```
 
+> The image sameersbn/mysql does not expose a database superuser,
+so you have to explicitly pass the database credentials here.
+
 Use the following database options when running the container:
 
 ```
@@ -105,8 +112,7 @@ Use the following database options when running the container:
 
 #### External database server
 
-If you already have a PostgreSQL or MySQL server around off docker you also can go with
-that.
+If you already have a PostgreSQL or MySQL server around off docker you also can go with that.
 Instead of linking docker containers you need to provide database hostname and port like so:
 
 ```