add a readme

README.md

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+# QOI for Allegro
+
+This is a simple liballeg plugin that gives Allegro the ability to decode QOI images, in both 24-bit and 32-bit formats.
+
+Some example images and a test program are provided. All of the code is in allegro_qoi.c. The header file declares only a single function `al_init_qoi()` which takes no parameters (same thing as `al_init_image_addon()`). Call it once when your program starts.
+
+If you want you can also just drop the `allegro_qoi.c` into your project and forward-declare it's init function once where it's used.
+
+This plugin does not handle saving QOI images yet.