Make Truss accessibility guide 1.0

[xtine]

PT IMPORTED STORY: https://www.pivotaltracker.com/story/show/171219462

Description Create a Truss opinionated guide for accessibility work based on existing guides

To do

• [x] Review existing guides and pull out pieces we like
• [x] Organize our own guide
• [ ] Publish guide to this repo to be linked to or included in other Truss material (playbooks, proposals, etc.)

Definition of done We have a published Truss accessibility guide that can be easily linked to from other Truss materials

[joshmfranklin]

Doc for outlining guide/checklist: https://docs.google.com/document/d/1zMS5-jOer8eigc2XwBbOd73UrCODRZJKxV3lPuHSdz8/edit