chore: document {devtag} in contributing guide #4236

Job	Run time
Build source R package	5m 41s
Check windows-latest (release)	10m 56s
Sanitizer	29m 32s
Check ubuntu-20.04 (release)	6m 40s
Coverage	51m 32s
Check ubuntu-20.04 (oldrel-1)	6m 33s
Check ubuntu-20.04 (oldrel-2)	6m 49s
Check ubuntu-20.04 (oldrel-3)	7m 31s
Check ubuntu-20.04 (oldrel-4)	8m 4s
	2h 13m 18s

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