Update README-wheel.md to document what's linked into pybindings

Since pybindings can be built with many configurations, it's important
to tell users what's actually present in the wheel.

Author: dbort

README-wheel.md

@@ -4,6 +4,22 @@ standard on-device iOS and Android mobile deployments. One of the main goals for
 ExecuTorch is to enable wider customization and deployment capabilities of the
 PyTorch programs.
 
+The `executorch` pip package is in alpha.
+* Required python version: 3.10
+* Compatible systems: Linux x86_64, macOS aarch64
+
+The prebuilt `executorch.extension.pybindings.portable_lib` module provides a way to
+run ExecuTorch `.pte` files, with some restrictions:
+* Only [core ATen
+  operators](https://pytorch.org/executorch/stable/ir-ops-set-definition.html)
+  are linked into the module
+* Some host-CPU-compatible backends are linked into the module:
+  * Linux: XNNPACK
+  * macOS: XNNPACK, MPS
+    * Core ML will be linked into the module in a future release. It is
+      generally available for ExecuTorch users in other contexts, but is not
+      linked into this module because of some automated build issues.
+
 Please visit the [ExecuTorch website](https://pytorch.org/executorch/) for
 tutorials and documentation. Here are some starting points:
 * [Getting