Document how to add a custom feed type

[arjanfrans]

I want to implement another feed type besides Google Shopping. I am currently searching through the source code on how to do that. It would be nice if this would be documented.

[loevgaard]

Create a service like this: https://github.com/Setono/SyliusFeedPlugin/blob/master/src/Resources/config/services/feed_type.xml#L6-L13

[loevgaard]

Would be great with a doc PR :)