readme: update mysql commands to create a database with real UTF-8 and give hint on user creation

[Trim]

Hello,

I'm creating again a development environment.

I've just seen that mysql commands currently advices to create the database with utf8 instead of utf8mb4 which is the real UTF-8 (for more details see this blog post which explain the first character set use a custom utf-8 on 3 bytes and the other uses the standard utf-8 on 4 bytes).

I took the opportunity too to add an example on how to create a user in mysql directly with SQL commands.

Thanks

[nono]

Thanks!

[Trim]

Mmmhh, sorry, that was a bad idea, with that setup I can't create the index

CREATE INDEX `index_oauth_access_grants_on_token` USING btree ON `oauth_access_grants` (`token`)

because I have this error while running db:setup:

Mysql2:Error: Specified key was too long: max key length is 767 bytes

That's strange, as I've seen in the database.yml file the character set is setted by default on utf8mb4.

I'll try tomorrow to create a Debian Jessie environment instead of Debian Stretch, because Stretch is using MariaDB.