upkeep: add Jackson as maintainer #18

Job	Run time
R Package Checks / codecov / test-coverage	1m 14s
R Package Checks / check-R-updates / updated-files	6s
R Package Checks / lintr / lint	1m 23s
R Package Checks / docs-check / docs-check	1m 38s
R Package Checks / r-cmd-check / macOS-latest (release)	2m 23s
R Package Checks / r-cmd-check / windows-latest (release)	5m 58s
R Package Checks / r-cmd-check / windows-latest (3.6)	5m 1s
R Package Checks / r-cmd-check / windows-latest (4.1)	4m 43s
R Package Checks / r-cmd-check / ubuntu-latest (release)	1m 44s
R Package Checks / r-cmd-check / ubuntu-latest (devel)	1m 55s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-1)	1m 35s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-2)	1m 38s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-3)	1m 54s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-4)	1m 47s
R Package Checks / version-check	0s
	32m 59s

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