Update maintenance note (#972)

Author: merrymercy

README.md

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+**Note: Alpa is not actively maintained currently. It is available as a research artifact. The core algorithm in Alpa has been merged into XLA, which is still being maintained. https://github.com/openxla/xla/tree/main/xla/hlo/experimental/auto_sharding**
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 <div align="center">
 <img src="https://github.com/alpa-projects/alpa/blob/main/docs/logo/alpa-logo-cropped.png" alt="logo" width="250"></img>
 <br></br>