docs: Update list of extensions

The README has a list of extensions with a brief description, but
the most recent additions haven't been added yet. Rectify that.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/352>

README.md

@@ -54,6 +54,11 @@ GSettings key.
 
      Changes application icons to always launch a new instance when activated.
 
+ * light-style
+
+    Changes the default shell style to "light", while still following the
+    system-wide "dark" preference.
+
  * native-window-placement
 
      An alternative algorithm for layouting the thumbnails in the windows overview, that
@@ -67,6 +72,14 @@ GSettings key.
 
      Adds a shortcut for resizing the focus window to a size that is suitable for GNOME Software screenshots. Ctrl + Alt + s cycles forwards through the available sizes and Ctrl + Alt + Shift + s cycles backwards.
 
+ * status-icons
+
+    Show (XEmbed) status icons in the top bar.
+
+ * system-monitor
+
+    Shows system usage information in the top bar.
+
  * user-theme
 
      Loads a shell theme from ~/.themes/<name>/gnome-shell.