Add a note about the Mac OS X version of package ui requiring Go 1.3 …

…to the README.

Updates issue 1.

README.md

@@ -7,6 +7,7 @@ ui aims to run on all supported versions of supported platforms. To be more prec
 
 * Windows: Windows 2000 or newer. The Windows backend uses package `syscall` and calls Windows DLLs directly, so does not rely on cgo.
 * Mac OS X: Mac OS X 10.6 (Snow Leopard) or newer. Objective-C dispatch is done by interfacing with libobjc directly, and thus this uses cgo.
+	* Note: you will need Go 1.3 or newer for this verison, as it uses a single .m file due to technical restrictions (read the comments in `bleh_darwin.m` for details), and earlier versions of Go do not auto-build .m files.
 * Other Unixes: The Unix backend uses GTK+, and thus cgo. It requires GTK+ 3.4 or newer; for Ubuntu this means 12.04 LTS (Precise Pangolin) at minimum. Check your distribution.
 
 ui itself has no outside Go package dependencies; it is entirely self-contained.