Minor updates to several README.md files

Just some formatting fixes and in some cases add links to XQuartz for
macOS Mountain Lion and beyond. More entries (probably many more) have
to have that link added but those will be found and fixed in time.

2012/dlowe/README.md

@@ -13,6 +13,11 @@
 make
 ```
 
+NOTE: this entry requires `X11/Xlib.h` header file and the X11 library to
+compile. macOS users running Mountain Lion and later will need to download and
+install [XQuartz](https://www.xquartz.org) in order to compile and run this
+entry.
+
 ### To run:
 
 ```sh
@@ -33,9 +38,6 @@ Use up and down keys to escape death.
 
 The judges lose playing this game. This entry wins!
 
-OS X users running Mountain Lion and later will need to download and install:
-
-	http://xquartz.macosforge.org/landing/
 
 Q: How did we get past the variadic macro problem on a Mac?