Add a note about installing libpng.

README

@@ -17,6 +17,15 @@ more like other UNIX tools.
   Installing RGBDS (UNIX)
 =========================
 
+RGBDS requires libpng and pkg-config to be installed.
+
+On Mac OS X, install them with [Homebrew](http://brew.sh/). On other Unixes,
+use the built-in package manager.
+
+You can test if they're installed by running "pkg-config --cflags libpng":
+if the output is a path, then you're good, and if it outputs an error then
+you need to install them via a package manager.
+
 To build the programs on a UNIX or UNIX-like system, just run in your terminal:
 
   make