add a work-in-progress article about how the Purdue CS boxes are set up

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+Customizing your login on Purdue CS computers (WIP)
+=============================================
+---
+date: "2014-04-21"
+---
+
+Most CS students at Purdue spend a lot of time on the lab boxes, but
+don't know a lot about them.  This document tries to fix that.
+
+The lab boxes all run Gentoo.
+
+## About GDM
+
+They run `gdm` (Gnome Display Manager) 2.20.11 for the login screen.
+This is an old version, and has a couple behaviors that are slightly
+different than new versions, but here are the important bits:
+
+System configuration:
+
+ - `/usr/share/gdm/defaults.conf` (lower precidence)
+ - `/etc/X11/gdm/custom.conf` (higher precidence)
+
+User configuration:
+
+ - `~/.dmrc` (more recent versions use `~/.desktop`, but Purdue boxes
+   aren't running more recent versions)
+
+## Purdue's GDM
+
+Now, `custom.conf` sets
+
+    BaseXsession=/usr/local/share/xsessions/Xsession
+    SessionDesktopDir=/usr/local/share/xsessions/
+
+This is important, because there are *multiple* locations that look
+like these files; I take it that they were used at sometime in the
+past.  Don't get tricked into thinking that it looks at
+`/etc/X11/gdm/Xsession` (which exists, and is where it would look by
+default).
+
+Values for `~/.dmrc:Session`
+ - `custom`: Executes `~/.xsession`
+ - `default`: Executes ~/.Xrc.cs
+ - `fvwm2`: ???
+ - `gnome`: ...
+ - `kde`: Displays a message saing KDE is no longer available
+ - `failsafe`: Runs a single xterm window (NOPE)
+