docs: add CONTRIBUTING.md file #386

Job	Run time
Build (1.65.0)	4m 31s
Build Libretro (1.70.0)	1m 25s
Build (1.66.0)	4m 23s
Build Libretro (1.71.0)	1m 20s
Build (1.67.0)	4m 27s
Build Libretro (1.72.0)	1m 20s
Build Libretro Android (1.71.0)	2m 47s
Build SDL (1.74.0)	7m 36s
Build Libretro (1.73.0)	1m 17s
Build (1.68.0)	4m 22s
Build SDL (1.75.0)	7m 22s
Build Libretro Android (1.72.0)	2m 55s
Build Libretro (latest)	1m 16s
Build (1.69.0)	4m 20s
Build SDL (1.76.0)	7m 36s
Build Libretro Android (1.73.0)	2m 48s
Build (1.70.0)	1m 24s
Build SDL (latest)	7m 31s
Build Libretro Android (latest)	2m 35s
Build (1.71.0)	1m 17s
Build WASM (1.74.0, 16)	3m 8s
Build (1.72.0)	1m 22s
Build WASM (1.75.0, 16)	3m 6s
Build (1.73.0)	1m 15s
Build WASM (1.76.0, 16)	3m 9s
Build (latest)	1m 9s
Build WASM (latest, 16)	3m 8s
	1h 28m 49s

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