update README

README.md

@@ -260,6 +260,18 @@ function onClickLogin() {
 onChangeProvider('oauth3.org');
 ```
 
+A user's e-mail can be passed into the clientParams object as `clientParams.subject`.
+
+To auto-populate the e-mail input of the login popup, make sure the input has the class `js-oauth3-email`.
+
+Example:
+```js
+if (clientParams.subject) {
+        $('.js-oauth3-email').val(clientParams.subject);
+        $('.js-authn-show').prop('disabled', false);
+      }
+```
+
 ### Compatibility with Frameworks and Libraries
 
 **jQuery**: