Update Reference.md

docs/Reference.md

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 This document lays out the current public properties and methods for the React Native WebView.
 
-> **Security Warning:** Currently, `onMessage` and `postMessage` do not allow specifying an origin. This can lead to cross-site scripting attacks if an unexpected document is loaded within a `WebView` instance. Please refer to the MDN documentation for [`Window.postMessage()`](https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage) for more details on the security implications of this.
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 ## Props Index
 
 - [`source`](Reference.md#source)