Improve README and add manual installation instructions

Fixes #1

README.md

@@ -1,8 +1,22 @@
 # certbot-systemd-nginx
-This is a small systemd unit for automating the Certbot certificate renewal for the nginx web server.
+This is a small `systemd` unit for automating the Certbot certificate renewal for the nginx web server.
 It runs monthly and simply executes
 
     certbot renew --standalone --keep-until-expiring
 
-As you can see, it uses the standalone authenticator, because Certbot nginx support is incomplete.
-This requires the web server to be stopped for a couple of seconds during renewal (once every two months).
+It uses the standalone authenticator, because Certbot nginx support is incomplete. This requires the
+web server to be stopped for a couple of seconds during each run (monthly, but can be overridden) and
+a bit more during renewal (once every two months).
+
+# Installation
+## Arch Linux
+On Arch Linux you can use the `certbot-systemd-nginx` AUR package.
+
+## Other Linux with systemd
+To use this on other Linux distributions that use `systemd`, you can copy the units to their usual
+location:
+
+    # cp certbot-nginx.{service,timer} /etc/systemd/system/
+    # systemctl daemon-reload
+    # systemctl start certbot-nginx.service # to start manually
+    # systemctl enable --now certbot-nginx.timer # to use the timer