add agetty service

The agetty service is an alternate way to manage gettys with agetty
under Linux which is separate from an external init system.

agetty-guide.md

@@ -0,0 +1,19 @@
+# Setting up the agetty service in OpenRC
+
+The agetty service is an OpenRC specific way to monitor and respawn a
+getty, using agetty, on Linux. To use this method, make sure you aren't
+spawning a getty manager for this port some other way (such as through
+sysvinit/inittab), then run the following commands as root.
+
+Note that [port] refers to the port you are spawning the getty on, and
+not the full path to it. For example, tty0 or ttyS0instead of /dev/tty0
+or /dev/ttyS0.
+tty0 or ttyS0, not the full path to it (for example, tty0 or ttyS0 and
+
+```
+# cd /etc/init.d
+# ln -s agetty agetty.[port]
+# cd /etc/conf.d
+# cp agetty agetty.[port]
+#rc-update add agetty.[port] [runlevel]
+```