Add section on setup for retina mockups.

Add section on setting up Photoshop for working on retina mocks using logical pixels rather than device pixels. This helps work in the same units as UI development and eases the transition between PSD and CSS.

Status: **Ready to merge**

Reviewers: @kpeatt @jeffkamo @jamesbull 
Ticket: N/A
Linked PRs: N/A

## Changes
- Add section on setup for retina mockups

## How to test-drive this PR
- Review the text
- Try the setup in Photoshop. Draw some shapes and resize using pt. Set some text in pt.

## Research resources
- I set this up initially following someone’s blog post but I can no longer find the link (although the Grid Settings caveat is original research)
Typo: "created a 2x resolution..." should probably be "created at 2x resolution"