Add install details

README.md

@@ -5,7 +5,7 @@ This is a very basic Firefox extension that modifies the canvas_session cookie s
 The extension expects that your Canvas instance is at a URL that begins with https://canvas.
 
 ## Installation
-idk I'm still working out how to make Firefox extensions.
+Download the .xpi file from Github releases (it's just a zip of the repository contents). Because it's not signed by Mozilla you'll need to set `xpinstall.signatures.required` to `false` in `about:config`. It's not clear to me if all Firefox variants even have this option, though.
 
 ## Security
 The Canvas mobile app already stays logged in forever, so this surely can't be too bad.