Extensibility RFC (openxla#1883)

This PR does not contain an actual RFC - it's merely tracking a
discussion on openxla-discuss:
https://groups.google.com/a/openxla.org/g/openxla-discuss/c/Ao5K8fvXoEk.
Once this RFC reaches an outcome, we'll update the status field in this
markdown file and merge this PR to document that.

rfcs/20230609-extensibility.md

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+# [RFC] StableHLO Extensibility
+
+Status: Approved<br/>
+Initial version: 6/9/2023<br/>
+Last updated: 3/8/2024<br/>
+Discussion thread: [openxla-discuss](https://groups.google.com/a/openxla.org/g/openxla-discuss/c/Ao5K8fvXoEk/m/OaddRrgyAgAJ).
+
+## Summary
+
+For full details and RFC discussion, see:
+[[RFC] StableHLO Extensibility](https://docs.google.com/document/d/1bSyyLA-p1F7KjZgjo563F1WFsPwcZc4eaH5WyQfbsi0/edit#heading=h.kfv34azf3j5k).
+
+In its role as a portability layer between ML frameworks and ML compilers,
+StableHLO provides a common vocabulary of well-understood ops along with
+compatibility guarantees for them. However, this all works only for a closed set
+of ops within the StableHLO dialect. In this document, we propose to offer a
+mechanism to create portable abstractions of StableHLO ops.