Add README to dist folder

Author: swinton

dist/README.md

@@ -0,0 +1,11 @@
+# :warning: IMPORTANT :warning:
+
+The contents of this `dist` folder are generated by a script, and should not be edited manually.
+
+To regenerate the contents of `dist`, run:
+
+```shell
+npm run prepare
+```
+
+This is typically run prior to every commit that modifies JavaScript code.