Update README with accessibility info.

README.md

@@ -18,3 +18,10 @@ This command will concatenate the source CSS files into a single CSS file, and
 then use PostCSS to add various vendor-prefixed properties. Open up
 `dist/index.html` to see a demo page of the various flexbox patterns in the
 browser.
+
+## Things to keep in mind
+
+_I don't recommend copy-pasting these examples directly into production code._
+I'm only trying to demonstrate different ways of using flexbox through these
+examples, so they may not incorporate some accessibility best practices (such as using semantic HTML5 elements and the `role` attribute). Before using this
+code in production you should make sure it meets your accessibility needs.