Add link to Google Chrome docs about Trial Tokens

Author: bershanskiy

proposals/manifest_trial_tokens.md

@@ -22,10 +22,11 @@ The manifest key to include origin and deprecation trial tokens.
 
 ### Objective
 
-This new manifest key empowers developers to use trial tokens in WebExtension
-service worker contexts, including the background service workers, which was
-not possible before. Also, this manifest key simplifies injection of tokens
-into all contexts with the extension's origin.
+This new manifest key empowers developers to use
+[trial tokens](https://developer.chrome.com/docs/web-platform/origin-trials)
+in WebExtension service worker contexts, including the background service workers,
+which was not possible before. Also, this manifest key simplifies injection of
+tokens into all contexts with the extension's origin.
 
 #### Use Cases