UX: Add a brief accessibility summary to the README (discourse#16602)

README.md

@@ -111,6 +111,10 @@ limitations under the License.
 
 Discourse logo and “Discourse Forum” ®, Civilized Discourse Construction Kit, Inc.
 
+## Accessibility
+
+To guide our ongoing effort to build accessible software we follow the [W3C’s Web Content Accessibility Guidelines (WCAG)](https://www.w3.org/TR/WCAG21/). If you'd like to report an accessibility issue that makes it difficult for you to use Discourse, email accessibility@discourse.org. For more information visit [discourse.org/accessibility](https://discourse.org/accessibility). 
+
 ## Dedication
 
 Discourse is built with [love, Internet style.](https://www.youtube.com/watch?v=Xe1TZaElTAs)