Update accessibility.md

Adding specific language about WCAG conformance.

Author: MelSumner

docs/concepts/accessibility.md

@@ -2,6 +2,8 @@
 
 ## Code Guide
 
+Our goal is to meet [WCAG](https://www.w3.org/WAI/standards-guidelines/aria/) [AA level conformance](https://www.w3.org/WAI/WCAG21/quickref/?currentsidebar=%23col_customize&levels=aaa&technologies=flash%2Csl) for all Ember.js websites.
+
 ### Iterative Improvements
 - It is 100% okay to ship code that can iterate to being a more inclusive experience- as long as it can still be used by people with assistive technology. 
 - It is not okay to ship code that actively breaks the experience for users with assistive technology.