Update readme

Readme.md

@@ -8,5 +8,19 @@ It simply redirects visits to e.g. `youtube.com/watch?v=12345` to `youtube.com/e
 
 To view a full-window video in its usual crufty context, click the extension's icon. It will then remain deactivated in that video's tab.
 
-Tested in Firefox (published [here](https://addons.mozilla.org/en-US/firefox/addon/youtubecinema/))
-and Chromium ([here](https://chrome.google.com/webstore/detail/youtubecinema/pdfgggfjbhlaacmggkejmfmjfdfdhahi)).
+
+## Status
+
+Works, ready to use.
+
+
+## Install
+
+The extension is available for Firefox and Chromium/Chrome/Brave/etc.
+
+For Firefox and its derivatives, find it [on Mozilla Addons][].
+
+For Chromium and its derivatives, unfortunately it is no longer available on the Chrome Web Store, as it “was in violation of branding guidelines”. To install it, [download the extension][], rename the file to end with `.zip` instead of `.xpi` so Chromium understands it. Then open `about:extensions`, turn on developer mode, then drag the zip file onto that page. (it may mention an error because of an unknown field in the manifest file, but should still work)
+
+[on Mozilla Addons]: https://addons.mozilla.org/en-US/firefox/addon/youtubecinema/
+[download the extension]: https://addons.mozilla.org/firefox/downloads/latest/youtubecinema