mention pypy in readme

README.md

@@ -28,6 +28,8 @@ pip install brownie
 pip install git+https://github.com/vyperlang/titanoboa
 ```
 
+Sometimes, using [pypy](https://www.pypy.org/download.html) can result in a substantial performance improvement for computation heavy contracts. `Pypy` can usually be used as a drop-in replacement for `CPython`.
+
 To get a performance boost for mainnet forking, install with the `forking-recommended` extra (`pip install "git+https://github.com/vyperlang/titanoboa#egg=titanoboa[forking-recommended]"`, or `pip install titanoboa[forking-recommended]`). This installs `plyvel` to cache RPC results between sessions, and `ujson` which improves json performance.
 
 If you are running titanoboa on a local [Vyper](https://github.com/vyperlang/vyper) project folder, you might need to run `python setup.py install` on your [Vyper](https://github.com/vyperlang/vyper) project if you encounter errors such as `ModuleNotFoundError: No module named 'vyper.version'`