Add v2 development warning to README #4055

Job	Run time
checks / pre-commit	31s
checks / readme-snippets	12s
checks / test (3.10, highest, ubuntu-latest)	45s
checks / test (3.11, highest, ubuntu-latest)	45s
checks / test (3.10, lowest-direct, windows-latest)	2m 19s
checks / test (3.12, lowest-direct, windows-latest)	3m 16s
checks / test (3.10, lowest-direct, ubuntu-latest)	48s
checks / test (3.12, lowest-direct, ubuntu-latest)	49s
checks / test (3.11, lowest-direct, windows-latest)	2m 27s
checks / test (3.12, highest, windows-latest)	3m 28s
checks / test (3.12, highest, ubuntu-latest)	53s
checks / test (3.13, lowest-direct, windows-latest)	2m 23s
checks / test (3.14, highest, windows-latest)	2m 21s
checks / test (3.14, highest, ubuntu-latest)	1m 13s
checks / test (3.13, highest, windows-latest)	3m 39s
checks / test (3.14, lowest-direct, windows-latest)	2m 19s
checks / test (3.13, lowest-direct, ubuntu-latest)	45s
checks / test (3.11, highest, windows-latest)	3m 28s
checks / test (3.13, highest, ubuntu-latest)	47s
checks / test (3.11, lowest-direct, ubuntu-latest)	46s
checks / test (3.10, highest, windows-latest)	2m 46s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 9s
	37m 49s

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