feature: the package installs the Router Manager, which is no longer a package of its own.

docs/system.md

@@ -0,0 +1,17 @@
+# System
+
+The System tab manages the router's operating system.
+
+## System updates
+
+- The panel lists the packages `apt-get upgrade` would upgrade, each with its installed and its
+  new version, as of apt's last update of its package lists.
+- **Check update** updates apt's package lists and lists the packages again.
+- **Upgrade** appears when a package can be upgraded; after a confirmation it runs
+  `apt-get upgrade -y` on the router and shows its output. It upgrades the installed packages
+  only: none is installed or removed, and a package that needs a new one is kept back.
+- Services restart during the upgrade; when `vpn-router` is upgraded, the page may not
+  answer for a while, and **Reload page** loads its new version.
+- A reboot is offered when a package asks for one, as after a new kernel.
+- Only one upgrade runs at a time: this one or the upgrade of `vpn-router` on the
+  [Overview](router.md) tab.