README: Clarify runlevel section a bit, new 'single' user mode

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

README.md

@@ -206,10 +206,18 @@ Support for SysV init-style [runlevels][5] is available, in the same
 minimal style as everything else in Finit.  The `[2345]` syntax can be
 applied to service, task, run, inetd, and TTY stanzas.
 
-All services in runlevel S(1) are started first, followed by the desired
-run-time runlevel.  Runlevel S can be started in sequence by using `run
-[S] cmd`.  Changing runlevels at runtime is done like any other init,
-e.g. <kbd>init 4</kbd>, but also using the more advanced `intictl` tool.
+Reserved runlevels are 0 and 6, halt and reboot, respectively just like
+SysV init.  Runlevel 1 can be configured freely, but is recommended to
+be kept as the system single-user runlevel since Finit will not start
+networking here.  The configured `runlevel NUM` from `/etc/finit.conf`
+is what Finit changes to after bootstrap, unless 'single' (or 'S') is
+given on the kernel cmdline, in which case runlevel 1 is started.
+
+All services in runlevel S(0) are started first, followed by the desired
+run-time runlevel.  Run tasks in runlevel S can be started in sequence
+by using `run [S] cmd`.  Changing runlevels at runtime is done like any
+other init, e.g. <kbd>init 4</kbd>, but also using the more advanced
+`intictl` tool.
 
 
 **Plugins**