Improve demo extension documentation (#49)

demo/readme.md

@@ -1,15 +1,36 @@
-Demo/test extension for `webext-permission-toggle`
+# Demo/test extension for `webext-permission-toggle`
 
-Run these 3 commands simultaneusly to manually test the module:
+First install `web-ext` globally:
+
+```sh
+npm i -g web-ext
+```
+
+To manually test the module, first run this the top-level directory of
+the repo:
 
 ```sh
 npm run watch
 ```
 
+Now leave that running and simultaneously run the following, which
+will run parcel to build the `mv2` and `mv3` targets defined in
+`package.json`:
+
 ```sh
-cd manual-test npm run watch
+npm run demo:watch
 ```
 
+Now you can launch various browser test combinations, e.g.
+
 ```sh
-web-ext run # Install it globally first
+web-ext run -s dist/mv2 -t firefox-desktop
+web-ext run -s dist/mv3 -t chromium
 ```
+
+Note that `package.json` specifies a default `web-ext` source
+directory of `dist/mv3`.
+
+In Chrome and Chromium, in order to view the background worker console
+debug messages, you'll need to enable Developer mode in
+`chrome://extensions/`.