create a document for mdev so people dont have to rtfs

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+-------------
+ MDEV Primer
+-------------
+
+For those of us who know how to use mdev, a primer might seem lame.  For
+everyone else, mdev is a weird black box that they hear is awesome, but can't
+seem to get their head around how it works.  Thus, a primer.
+
+-----------
+ Basic Use
+-----------
+
+Mdev has two primary uses: initial population and dynamic updates.  Both
+require sysfs support in the kernel and have it mounted at /sys.  For dynamic
+updates, you also need to have hotplugging enabled in your kernel.
+
+Here's a typical code snippet from the init script:
+[1] mount -t sysfs sysfs /sys
+[2] echo /bin/mdev > /proc/sys/kernel/hotplug
+[3] mdev -s
+
+Of course, a more "full" setup would entail executing this before the previous
+code snippet:
+[4] mount -t tmpfs mdev /dev
+[5] mkdir /dev/pts
+[6] mount -t devpts devpts /dev/pts
+
+The simple explanation here is that [1] you need to have /sys mounted before
+executing mdev.  Then you [2] instruct the kernel to execute /bin/mdev whenever
+a device is added or removed so that the device node can be created or
+destroyed.  Then you [3] seed /dev with all the device nodes that were created
+while the system was booting.
+
+For the "full" setup, you want to [4] make sure /dev is a tmpfs filesystem
+(assuming you're running out of flash).  Then you want to [5] create the
+/dev/pts mount point and finally [6] mount the devpts filesystem on it.
+
+-------------
+ MDEV Config   (/etc/mdev.conf)
+-------------
+
+Mdev has an optional config file for controlling ownership/permissions of
+device nodes if your system needs something more than the default root/root
+660 permissions.
+
+The file has the format:
+	<device regex> <uid>:<gid> <octal permissions>
+For example:
+	hd[a-z][0-9]* 0:3 660
+
+The config file parsing stops at the first matching line.  If no line is
+matched, then the default of 0:0 660 is used.  To set your own default, simply
+create your own total match like so:
+	.* 1:1 777
+
+If you also enable support for executing your own commands, then the file has
+the format:
+	<device regex> <uid>:<gid> <octal permissions> [<@|$|*> <command>]
+The special characters have the meaning:
+	@ Run after creating the device.
+	$ Run before removing the device.
+	* Run both after creating and before removing the device.
+
+The command is executed via the system() function (which means you're giving a
+command to the shell), so make sure you have a shell installed at /bin/sh.
+
+For your convenience, the shell env var $MDEV is set to the device name.  So if
+the device 'hdc' was matched, MDEV would be set to "hdc".