Improved the contrubiting docs #1315

Job	Run time
build (--test-successful-rustc --nb-parts 2 --current-part 0)	47s
build (--mini-tests)	47s
build (--test-successful-rustc --nb-parts 2 --current-part 1)	50s
build (--std-tests)	49s
build (--test-libcore)	48s
success_gcc12	2s
	4m 3s

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