Add notice about arrow2 being unmaintained to README (#1606) #1

Job	Run time
Clippy	1m 5s
Feature coverage	13s
fmt	2m 43s
cross (wasm32-unknown-unknown)	2m 51s
SIMD	2m 10s
Miri	2m 27s
cross (i686-unknown-linux-gnu)	2m 55s
cross (powerpc-unknown-linux-gnu)	2m 54s
MIRI on IO IPC	2m 44s
MIRI on IO IPC mmaping	2m 29s
Test full	18s
cross (powerpc64-unknown-linux-gnu)	2m 56s
cross (mips-unknown-linux-gnu)	2m 55s
windows-latest	1m 12s
cross (arm-linux-androideabi)	2m 54s
macos-latest	1m 6s
	33m 52s

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