Added contributing guide. #8

Job	Run time
buildbot / check-if-allowed	1s
buildbot / print	1m 2s
buildbot / asan (njs)	4m 4s
buildbot / msan	5m 10s
buildbot / asan (quickjs)	5m 9s
buildbot / test (alpine-3.19, amd64)	5m 56s
buildbot / test (alpine-3.19, arm64)	3m 56s
buildbot / test (amazonlinux-2, amd64)	3m 26s
buildbot / test (amazonlinux-2, arm64)	2m 34s
buildbot / test (amazonlinux-2023, amd64)	8m 7s
buildbot / test (amazonlinux-2023, arm64)	5m 55s
buildbot / test (debian-11, amd64)	4m 12s
buildbot / test (debian-11, arm64)	3m 8s
buildbot / test (debian-12, amd64)	5m 13s
buildbot / test (debian-12, arm64)	3m 43s
buildbot / test (freebsd-14, amd64)	3m 58s
buildbot / test (rhel-7, amd64)	2m 20s
buildbot / test (rhel-8, amd64)	4m 17s
buildbot / test (rhel-8, arm64)	2m 45s
buildbot / test (rhel-9, amd64)	7m 53s
buildbot / test (rhel-9, arm64)	5m 34s
buildbot / test (sles-12, amd64)	2m 22s
buildbot / test (ubuntu-20.04, amd64)	4m 27s
buildbot / test (ubuntu-20.04, arm64)	3m 17s
buildbot / test (ubuntu-22.04, amd64)	7m 32s
buildbot / test (ubuntu-22.04, arm64)	5m 48s
buildbot / test (ubuntu-23.10, amd64)	8m 7s
buildbot / test (ubuntu-23.10, arm64)	6m 15s
buildbot / test (debian-12, x86)	5m 28s
	2h 11m 39s

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