Point to experimental branch.

README.md

 # Pybind11 bindings for the Abseil C++ Common Libraries
+
+## WORK IN PROGRESS
+
+We are currently working on automatically syncing this repo with
+Google-internal developments. You can inspect the current state on the
+experimental branch:
+
+https://github.com/pybind/pybind11_abseil/tree/experimental
+
+**IMPORTANT**: The `experimental` branch may be purged and re-populated at
+any moment. Do NOT use the `experimental` branch for developments, and
+do NOT `git pull`. To update, always use:
+```
+git fetch
+git checkout experimental
+git reset --hard experimental
+```