friendlier readme for yimg

Former-commit-id: 17e1056354b11944cc2897776c32ba3d936b849e

README.md

@@ -155,6 +155,9 @@ To build the a static YImg library on Mac OS X issue:
     cd external/yimg
     make
 
+You may need to install libpng. Systems with X11 might find this already
+installed at `/usr/X11/lib`.
+
 
 ### Windows (Experimental) ###
 To build a static library (.lib) on windows, open Visual Studio 2010.