doc: correct #usage guidance

We erroneously instructed users to use `notify_fifo_script`, which is
wrong.

Update the section to note the need for a running kdnotify daemon, and
point at the systemd files we provide to that end

README.md

@@ -9,15 +9,16 @@ prone handling of parsing and processing each event individually.
 
 ## Usage
 
-The primary usage of this binary requires two things:
+The primary usage of this binary requires three things:
 
 1. A configuration file in `/etc/kdnotify/config.yaml`
-2. Enabling Keepalived to use this project for notifications via:
+2. A running kdnotify service, see [kdnotify.socket](./systemd/kdnotify.socket)
+3. Enabling Keepalived to use this project for notifications via:
 
 ```conf
 # Or in any other block where vrrp_notify_fifo_script is legal
-global {
-  vrrp_notify_fifo_script "/bin/kdnotify watch" [username [groupname]]
+global_defs {
+  notify_fifo /run/kdnotify.fifo [username [groupname]]
 }
 ```
 
@@ -26,12 +27,13 @@ global {
 You can run `make help` in the repo root to get a list of available targets, but these
 are the most useful:
 
-| Target    | Description                                                  |
-|:----------|:-------------------------------------------------------------|
-| `build`   | Build the binary to `build/kdnotify`                         |
-| `check`   | Run project checks and lints                                 |
-| `install` | Install project to the provided Prefix= (defaults to `/bin`) |
-| `clean`   | Cleans build directory                                       |
+| Target      | Description                             |
+|:------------|:----------------------------------------|
+| `build`     | Build the binary to `build/kdnotify`    |
+| `check`     | Run project checks and lints            |
+| `install`   | Install the project to the local system |
+| `clean`     | Cleans build directory                  |
+| `configure` | Configures build time directives        |
 
 Make sure you run `check` before merging to `master`.