How do I add Custom fields to the Order Information Checkout page in Commerce 2?
I found the answer to this, it's not very obvious at the moment due to the lack of documentation but it's blinding simple when you know how.

Install the new profile module.
https://www.drupal.org/project/profile

Edit the page:

configuration &gt; people &gt; user profile


Here you can add commerce2 user pane or new fields.

For me I needed to make a new Content type form that the user could create and fill out as part of the checkout process, so I used the inline entity form. 

Really you could use any field collections or paragraph's to add to the user fields as I notice there is a new patch to support field group and paragrahs.

I hope that helps someone else looking for this answer.