Update README.md

Author: connorjward

README.md

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+# ARCHIVED
+
+**The plan for `pyop3` is for it to be merged into Firedrake directly. Please see the `connorjward/pyop3` and `connorjward/pyop3-develop` branches there.**
+
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
 [![Imports: isort](https://img.shields.io/badge/%20imports-isort-%231674b1?style=flat&labelColor=ef8336)](https://pycqa.github.io/isort/)