Added more documentation

INSTALL.md

@@ -5,13 +5,15 @@
 - Xlib
 - Xt
 - Motif
+- MWM
 - A C compiler
 - brightnessctl (for xmbrightness)
+- xwallpaper
 
 Installing via APK:
 
 ```
-# apk add libx11 libx11-dev libxt libxt-dev motif motif-dev clang
+# apk add libx11 libx11-dev libxt libxt-dev motif motif-dev mwm xwallpaper clang
 ```
 
 ## libXmd
@@ -33,3 +35,20 @@ subdirectories:
 # cd <application>
 # ./build.sh install
 ```
+
+## Replicants
+
+Xmd provides something called replicants, which are dynamically loaded widgets
+that can be placed inside of xmpanel. They can be installed using `./build.sh`
+in their respective subdirectories:
+
+```
+# cd replicants/<replicant>
+# ./build.sh install
+```
+
+## Session
+
+Xmd includes a startup script and assorted "glue code" to facilitate using it as
+a desktop environment. It can be installed using `./install.sh` in the `session`
+directory.