Add instruction in README for how to upgrade slither #5749

Job	Run time
tests (ubuntu-latest, 3.8, data_dependency)	29s
tests (ubuntu-latest, 3.8, path_filtering)	29s
tests (ubuntu-latest, 3.8, erc)	31s
tests (ubuntu-latest, 3.8, find_paths)	37s
tests (ubuntu-latest, 3.8, flat)	30s
tests (ubuntu-latest, 3.8, interface)	33s
tests (ubuntu-latest, 3.8, printers)	1m 5s
tests (ubuntu-latest, 3.8, slither_config)	31s
tests (ubuntu-latest, 3.8, upgradability)	51s
tests (ubuntu-latest, 3.12, data_dependency)	29s
tests (ubuntu-latest, 3.12, path_filtering)	29s
tests (ubuntu-latest, 3.12, erc)	26s
tests (ubuntu-latest, 3.12, find_paths)	28s
tests (ubuntu-latest, 3.12, flat)	35s
tests (ubuntu-latest, 3.12, interface)	42s
tests (ubuntu-latest, 3.12, printers)	1m 6s
tests (ubuntu-latest, 3.12, slither_config)	33s
tests (ubuntu-latest, 3.12, upgradability)	48s
	11m 12s

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