Update README.md

Author: steipete

README.md

@@ -9,6 +9,9 @@ See our blog post announcements for details:
 * [React Native UI Component for Android](https://pspdfkit.com/blog/2018/react-native-ui-component-for-android/) ([See Android](https://github.com/PSPDFKit/react-native#android))
 * [PSPDFKit for Windows UWP with React Native](https://pspdfkit.com/2018/introducing-pspdfkit-windows/#react-native-for-windows-support) ([See Windows UWP](https://github.com/PSPDFKit/react-native#windows-uwp))
 
+The [PSPDFKit SDK](https://pspdfkit.com/) is a framework that allows you to view, annotate, sign, and fill PDF forms on iOS, Android, Windows, macOS, and Web.
+[PSPDFKit Instant](https://pspdfkit.com/instant) adds real-time collaboration features to seamlessly share, edit, and annotate PDF documents.
+
 ### iOS
 
 #### Requirements