Add Authentication section

Author: jdnichollsc

README.md

@@ -142,6 +142,10 @@ import InAppBrowser from 'nativescript-inappbrowser'
 ...
 ```
 
+### Authentication
+
+Using in-app browser tabs (like SFAuthenticationSession/ASWebAuthenticationSession and Android Custom Tabs) where available. Embedded user-agents, known as web-views (like UIWebView and WKWebView), are explicitly not supported due to the usability and security reasons documented in [Section 8.12 of RFC 8252](https://tools.ietf.org/html/rfc8252#section-8.12).
+
 ## Credits 👍
 * **React Native InAppBrowser:** [InAppBrowser for React Native](https://github.com/proyecto26/react-native-inappbrowser)