Add instructions for creating a new account/app in Okta

Author: Matt Raible

README.md

@@ -348,6 +348,12 @@ await appAuth.revokeToken(refreshedState.refreshToken);
 
 Full support out of the box.
 
+> If you're using Okta and want to add App Auth to your React Native application, you'll need an application to authorize against. If you don't have an Okta Developer account, [you can signup for free](https://developer.okta.com/signup/).
+>
+> Log in to your Okta Developer account and navigate to **Applications** > **Add Application**. Click **Native** and click the **Next** button. Give the app a name you’ll remember (e.g., `React Native`), select `Refresh Token` as a grant type, in addition to the default `Authorization Code`. Copy the **Login redirect URI** (e.g., `com.oktapreview.dev-158606:/callback`) and save it somewhere. You'll need this value when configuring your app.
+>
+> Click **Done** and you'll see a client ID on the next screen. Copy the redirect URI and clientId values into your App Auth config.
+
 ```js
 const scopes = ["openid", "profile"];
 const appAuth = new AppAuth({