Mention upcoming How to Extend React Native API blog post in “Announcements”

Author: RadAzzouz

README.md

@@ -9,6 +9,7 @@ This wrapper requires a valid license of PSPDFKit. Licenses are per platform. Yo
 * [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/blog/2018/introducing-pspdfkit-windows/#react-native-for-windows-support) ([See Windows UWP](https://github.com/PSPDFKit/react-native#windows-uwp))
+* [How to Extend React Native API](https://pspdfkit.com/blog/2018/how-to-extend-react-native-api/)
 
 #### PSPDFKit