Update README.md

Author: britzl

README.md

@@ -7,6 +7,9 @@ https://github.com/defold/extension-webview/archive/master.zip
 
 We recommend using a link to a zip file of a [specific release](https://github.com/defold/extension-webview/releases).
 
+## Usage
+Full documentation: https://www.defold.com/extension-webview
+
 ## API reference
 
 https://defold.com/extension-webview/api/