Merge pull request rails#8469 from kytrinyx/explicit-mysql-instructions

Include command to create mysql user

[ci skip]

Author: rafaelfranca

guides/source/development_dependencies_install.md

@@ -145,6 +145,9 @@ We need first to delete `.bundle/config` because Bundler remembers in that file
 In order to be able to run the test suite against MySQL you need to create a user named `rails` with privileges on the test databases:
 
 ```bash
+$ mysql -uroot -p
+
+mysql> CREATE USER 'rails'@'localhost';
 mysql> GRANT ALL PRIVILEGES ON activerecord_unittest.*
        to 'rails'@'localhost';
 mysql> GRANT ALL PRIVILEGES ON activerecord_unittest2.*