Update MySQL readme to document

Adds a section in the docs for the new capabilities added by docker-library/mysql#237.

Author: TFenby

mysql/README.md

@@ -147,6 +147,14 @@ This is an optional variable. Set to `yes` to generate a random initial password
 
 Sets root (*not* the user specified in `MYSQL_USER`!) user as expired once init is complete, forcing a password change on first login. *NOTE*: This feature is supported on MySQL 5.6+ only. Using this option on MySQL 5.5 will throw an appropriate error during initialization.
 
+## Docker Secrets
+
+As an alternative to passing sensitive information via environemnt variables, \_FILE may be appended to the previously listed environment variables, causing the initialization script to load the values for those variables from files present in the container. In particular, this can be used to load passwords from Docker secrets stored in /run/secrets/<secret_name> files. For example:
+
+```console
+$ docker run --name some-mysql -e MYSQL_ROOT_PASSWORD_FILE=/run/secrets/mysql-root -d mysql:tag
+```
+
 # Initializing a fresh instance
 
 When a container is started for the first time, a new database with the specified name will be created and initialized with the provided configuration variables. Furthermore, it will execute files with extensions `.sh`, `.sql` and `.sql.gz` that are found in `/docker-entrypoint-initdb.d`. Files will be executed in alphabetical order. You can easily populate your mysql services by [mounting a SQL dump into that directory](https://docs.docker.com/engine/tutorials/dockervolumes/#mount-a-host-file-as-a-data-volume) and provide [custom images](https://docs.docker.com/reference/builder/) with contributed data. SQL files will be imported by default to the database specified by the `MYSQL_DATABASE` variable.