chore(docs): Added additional note to `source` parameter (#621)

Added a note about the baseUrl usage since this has been the cause of some filed issues e.g. https://github.com/react-native-community/react-native-webview/issues/526#issuecomment-498067637

docs/Reference.md

 
 **Static HTML**
 
-_Note that using static HTML requires the WebView property [originWhiteList](Reference.md#originWhiteList) to `['*']`._
+_Note that using static HTML requires the WebView property [originWhiteList](Reference.md#originWhiteList) to `['*']`. For some content, such as video embeds (e.g. Twitter or Facebook posts with video), the baseUrl needs to be set for the video playback to work_
 
 - `html` (string) - A static HTML page to display in the WebView.
 - `baseUrl` (string) - The base URL to be used for any relative links in the HTML.