Missing documentation for custom migration template

[Holicz]

The possibility of setting custom_template is not referenced in the docs, only in the docs for DoctrineMigrationsBundle.

Also, I was able to find only one article on this matter which is for version 2.x and is not working anymore since <version> was removed and <className> was added.

I think it would be best to add a new chapter with this information + information about all possible tags + example of a template.

I could provide a PR if agreed this part of documentation should be added.

[SenseException]

We appreciate if missing documentation can be added.

[Holicz]

Ok, thank you for confirmation.

Should I add a new chapter to the sidebar or insert it in some existing chapter?

[SenseException]

You can insert it on an existing chapter of the config.