add a consistent set of text about X11

2001/williams/README.md

@@ -4,6 +4,10 @@
     make
 ```
 
+NOTE: this entry requires the `X11/Xlib.h` header file and the X11 library to
+compile.  For more information see the
+FAQ on "[X11](../../faq.html#X11)".
+
 
 ### Bugs and (Mis)features: