Add instruction in README for how to upgrade slither #2033

Job	Run time
tests (ubuntu-latest, unit, 3.8)	54s
tests (ubuntu-latest, unit, 3.12)	42s
tests (ubuntu-latest, integration, 3.8)	3m 14s
tests (ubuntu-latest, integration, 3.12)	3m 20s
tests (ubuntu-latest, tool, 3.8)	53s
tests (ubuntu-latest, tool, 3.12)	50s
tests (windows-2022, unit, 3.8)	1m 33s
tests (windows-2022, unit, 3.12)	1m 41s
tests (windows-2022, integration, 3.8)	2m 49s
tests (windows-2022, integration, 3.12)	3m 16s
tests (windows-2022, tool, 3.8)	3m 23s
tests (windows-2022, tool, 3.12)	3m 31s
coverage	16s
	26m 22s

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