Add link to the metadata format reference table

README.md

@@ -293,6 +293,7 @@ Resources
 
 - [This blog post](http://w-shadow.com/blog/2010/09/02/automatic-updates-for-any-plugin/) has more information about the update checker API. *Slightly out of date.*
 - [Debug Bar](https://wordpress.org/plugins/debug-bar/) - useful for testing and debugging the update checker.
+- [Update format reference](https://docs.google.com/spreadsheets/d/1eOBbW7Go2qEQXReOOCdidMTf_tDYRq4JfegcO1CBPIs/edit?usp=sharing) - describes all fields supported by the JSON-based update information format used by the update checker. Only covers plugins. Themes use a similar but more limited format.
 - [Securing download links](http://w-shadow.com/blog/2013/03/19/plugin-updates-securing-download-links/) - a general overview.
 - [A GUI for entering download credentials](http://open-tools.net/documentation/tutorial-automatic-updates.html#wordpress)
 - [Theme Update Checker](http://w-shadow.com/blog/2011/06/02/automatic-updates-for-commercial-themes/) - an older, theme-only variant of this update checker.