Add v2 development warning to README (#1862) #555

Job	Run time
checks / readme-snippets	12s
checks / test (3.11, lowest-direct, ubuntu-latest)	48s
checks / test (3.10, lowest-direct, windows-latest)	2m 23s
checks / test (3.13, lowest-direct, windows-latest)	2m 39s
checks / test (3.11, highest, windows-latest)	2m 33s
checks / test (3.12, highest, windows-latest)	3m 38s
checks / test (3.12, lowest-direct, windows-latest)	3m 21s
checks / test (3.10, lowest-direct, ubuntu-latest)	48s
checks / test (3.10, highest, ubuntu-latest)	51s
checks / test (3.11, highest, ubuntu-latest)	46s
checks / test (3.13, lowest-direct, ubuntu-latest)	46s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 12s
checks / test (3.14, highest, ubuntu-latest)	1m 8s
checks / test (3.14, lowest-direct, windows-latest)	4m 16s
checks / test (3.12, highest, ubuntu-latest)	55s
checks / test (3.13, highest, ubuntu-latest)	44s
checks / test (3.11, lowest-direct, windows-latest)	2m 29s
checks / test (3.14, highest, windows-latest)	2m 31s
checks / pre-commit	39s
checks / test (3.13, highest, windows-latest)	2m 42s
checks / test (3.10, highest, windows-latest)	2m 24s
checks / test (3.12, lowest-direct, ubuntu-latest)	58s
	38m 43s

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