Intro in documentation needs update

[jblotus]

The documentation for 3.0 has not been updated - I was going to create a pull request to fix it but it references a phar file for v2. Is there a phar file for version 3? Is this still a recommended way to use this package?

https://github.com/doctrine/migrations/blob/3.0.1/docs/en/reference/introduction.rst