doc: add some info about keventd, what it does

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/config.md

@@ -26,6 +26,7 @@ Configuration
   * [/etc/finit.conf](#etcfinitconf)
   * [/etc/finit.d](#etcfinitd)
 * [Watchdog](#watchdog)
+* [keventd](#keventd)
 
 
 Introduction
@@ -704,3 +705,19 @@ the only option is to remove `/libexec/finit/watchdogd` or build without
 it at configure time.
 
 [bootstrap]: bootstrap.md
+
+
+keventd
+-------
+
+The kernel event daemon bundled with Finit is a simple uevent monitor
+for `/sys/class/power_supply`.  It provides the `sys/pwr/ac` condition,
+which can be useful to prevent power hungry services like anacron to run
+when a laptop is only running on battery, for instance.
+
+Since keventd is not an integral part of Finit yet it is not enabled by
+default.  Enable it using `./configuure --with-keventd`.  The bundled
+contrib build scripts for Debian, Alpine, and Void have this enabled.
+
+This daemon is planned to be extended with monitoring of other uevents,
+patches and ideas are welcome in the issue tracker.