[a11y] Exception list a11y docs

[dpersing]

WHY are these changes introduced?

Adds accessibility guidance for the exception list component, to appear in polaris-react docs and in the style guide.

WHAT is this pull request doing?

• Adds accessibility information to the component README.md
• Adds an entry to the documentation section of UNRELEASED.md

How to 🎩

1. Check out master from polaris-styleguide
2. In another Terminal tab or window, check out this branch from polaris-react and run the instructions for testing in a consuming project
3. In the polaris-styleguide tab, run dev up && dev start
4. View changes after examples and props: https://polaris.myshopify.io/components/lists-and-tables/exception-list (web only)

Screenshots

Web

[sadiesaurus]

Tiny comment