Add link to od-default-disabled plugin

Author: westonruter

plugins/optimization-detective/docs/extensions.md

@@ -52,3 +52,4 @@ For development and debugging, also on GitHub:
 * [Optimization Detective Store Query Vars](https://github.com/westonruter/od-store-query-vars): Stores the Query Vars with a URL Metric in the Optimization Detective plugin. This is useful for debugging URL Metrics, in particular what the slug was computed from.
 * [Optimization Detective Store User Agent](https://github.com/westonruter/od-store-user-agent): Stores the User Agent with a URL Metric in the Optimization Detective plugin. This is useful for debugging URL Metrics, in particular to understand what device has a given viewport's dimensions.
 * [Optimization Detective Dev Mode](https://github.com/westonruter/od-dev-mode): Adds filters to facilitate development of the Optimization Detective plugin.
+* [Optimization Detective Default Disabled](https://github.com/westonruter/od-default-disabled): Disables functionality in Optimization Detective by default unless the `?optimization_detective_enabled=1` query parameter is present. This allows you for troubleshooting issues on a live site without visitors seeing a broken experience.