Added a basic README.

README.md

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+# libui: a portable GUI library for C
+
+This README is being written.
+
+## Requirements
+
+* Windows: Windows Vista or newer
+** At present, you must provide a manifest file
+* Unix: GTK+ 3.4 or newer
+* OS X: OS X 10.7 or newer
+
+## Documentation
+
+Needs to be written. Consult ui.h and the examples for details for now.
+
+## Screenshots
+
+From examples/controlgallery:
+
+![Windows](examples/controlgallery/windows.png)
+
+![Unix](examples/controlgallery/unix.png)
+
+![OS X](examples/controlgallery/darwin.png)