Add warning about API instability.

PiperOrigin-RevId: 630395902

README.md

@@ -50,6 +50,19 @@ each useable independently:
 Documentation on Penzai can be found at
 [https://penzai.readthedocs.io](https://penzai.readthedocs.io).
 
+> [!WARNING]
+> Penzai's API is currently unstable and may change in future releases.
+>
+> In particular, the way Penzai handles parameter initialization, parameter
+> sharing, and local mutable state in `penzai.nn` and
+> `penzai.data_effects` is likely to be simplified in the future.
+> Some internal details of the `treescope` pretty-printer intermediate
+> representation may also change to make it easier to extend and configure.
+>
+> Projects that use Penzai's neural network components or model implementations,
+> or that define their own handlers for `treescope`, are encouraged to pin the
+> `0.1.x` release series (e.g. `penzai>=0.1,<0.2`) to avoid breaking changes.
+
 
 ## Getting Started
 

docs/index.rst

@@ -61,6 +61,20 @@ useable independently:
 These components are described in more detail in the guides in the left
 sidebar.
 
+.. warning::
+   Penzai's API is currently unstable and may change in future releases.
+
+   In particular, the way Penzai handles parameter initialization, parameter
+   sharing, and local mutable state in `penzai.nn` and
+   `penzai.data_effects` is likely to be simplified in the future.
+   Some internal details of the `penzai.treescope` pretty-printer intermediate
+   representation may also change to make it easier to extend and configure.
+
+   Projects that use Penzai's neural network components or model
+   implementations, or that define their own handlers for ``treescope``, are
+   encouraged to pin the ``0.1.x`` release series (e.g. ``penzai>=0.1,<0.2``)
+   to avoid breaking changes.
+
 Getting Started
 ---------------