PR #33513: [docs] note mlir-hlo deprecation

Imported from GitHub PR #33513

Closes #33472, by noting that the project will be deprecated.  Will reduce the chance of folk exploring the project without this knowledge.
Copybara import of the project:

--
f82b010 by Perry Gibson <[email protected]>:

docs: note mlir-hlo deprecation

Merging this change closes #33513

COPYBARA_INTEGRATE_REVIEW=#33513 from Wheest:pez/mlirHLO f82b010
PiperOrigin-RevId: 827888690

Author: Wheest

xla/mlir_hlo/README.md

@@ -1,5 +1,9 @@
 # MLIR-HLO: A Standalone "HLO" MLIR-based Compiler
 
+> Note:
+> [this project is deprecated](https://groups.google.com/a/openxla.org/g/openxla-discuss/c/Mppuv1Edv1s),
+> and will be removed in the future.
+
 The code here exists in two places:
 
 *   https://github.com/openxla/xla/tree/main/xla/mlir_hlo: