Update tested up to version and add note about HTTPS in core

Closes #666

README.md

@@ -7,7 +7,7 @@ WordPress feature plugin to bring Progressive Web App (PWA) capabilities to Core
 **Contributors:** [google](https://profiles.wordpress.org/google), [xwp](https://profiles.wordpress.org/xwp), [westonruter](https://profiles.wordpress.org/westonruter), [albertomedina](https://profiles.wordpress.org/albertomedina)  
 **Tags:** [pwa](https://wordpress.org/plugins/tags/pwa), [progressive web apps](https://wordpress.org/plugins/tags/progressive-web-apps), [service workers](https://wordpress.org/plugins/tags/service-workers), [web app manifest](https://wordpress.org/plugins/tags/web-app-manifest)  
 **Requires at least:** 5.5  
-**Tested up to:** 5.7  
+**Tested up to:** 5.9  
 **Stable tag:** 0.6.0  
 **License:** [GPLv2 or later](http://www.gnu.org/licenses/gpl-2.0.html)  
 **Requires PHP:** 5.6
@@ -37,7 +37,7 @@ In general a PWA depends on the following technologies to be available:
 * [Web App Manifest](https://developer.mozilla.org/en-US/docs/Web/Manifest)
 * [HTTPS](https://en.wikipedia.org/wiki/HTTPS)
 
-This plugin serves as a place to implement support for these in WordPress with the intention of being proposed for core merge, piece by piece.
+This plugin serves as a place to implement support for these in WordPress with the intention of being proposed for core merge, piece by piece. Case in point, the HTTPS functionality from this plugin has largely been made part of Core: [Improved HTTPS detection and migration in WordPress 5.7](https://make.wordpress.org/core/2021/02/22/improved-https-detection-and-migration-in-wordpress-5-7/).
 
 This feature plugin is _not_ intended to obsolete the other plugins and themes which turn WordPress sites into PWAs. Rather, this plugin is intended to provide the PWA building blocks and coordination mechanism for these themes and plugins to not reinvent the wheel and also to not conflict with each other. For example, a theme that implements the app shell model should be able to extend the core service worker while a plugin that provides push notifications should be able to do the same. Themes and plugins no longer should have to each create a service worker on their own, something which is inherently problematic because only one service worker can be active at a time: only one service worker can win. If you are developing a plugin or theme that includes a service worker, consider relying on this PWA plugin, or at least only use the built-in implementation as a fallback for when the PWA plugin is not available.