[GitHub] Revise CONTRIBUTING.md for clarity and updates #1322

Job	Run time
filter_build	13s
build-on / display-inputs	4s
build-on / sh-ubuntu_gcc_release	1m 8s
build-on / sh-fedora_clang_release	1m 4s
build-on / sh-macos_clang_release	36s
	3m 5s

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