add a README with screenshots

README.md

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+# libgral
+
+libgral is a low-level abstraction layer exposing a unified C API to open windows, receive keyboard and mouse events, draw text, images, and vector graphics, access the clipboard, play PCM audio, and receive MIDI events on Linux, macOS, and Windows.
+
+![Linux screenshot](https://github.com/user-attachments/assets/5854a18c-8a36-41e5-bb98-bf025266fe46)
+
+![macOS screenshot](https://github.com/user-attachments/assets/6a34fc32-52c2-4c3f-96ee-5f1bf3e2fc95)