Add migration guide

[samanpwbb]

@katydecorah for review

closes https://github.com/mapbox/assembly/issues/1004

[katydecorah]

I've been using this guide on dr-ui. It's working well! One question:

What do you think about a section to call out sizing and color changes? This would remind me to check these elements to make sure they work well with my page.

Maybe something like:

Changes to element sizing, color, and defaults

• The height of toggle is now 30px, down from 36px and the height of toggle-s is now 18px, down from 24px.
• The default form control colors is gray, originally blue, for all form types.
• The typographic scale has been updated. Notably txt-m is now 16px, up from 15px and many headings have a smaller font-size.
• Shadow classes now have a y-offset.
• The default select is now borderless. To add a border, use select--stroke.

[samanpwbb]

@katydecorah i pulled in those changes. Back to you again!