README: mention the wabt-package Python package (#2906)

Adds a short note to "Installing prebuilt binaries" about
`wabt-package`, an unofficial package on PyPI that ships the tools and
the `wasm2c` runtime files as prebuilt wheels, with versions that follow
WABT's. It is maintained outside this repository.

Closes: #2899

README.md

diff --git a/README.md b/README.md
index 6d1bf2b..00c9357 100644
--- a/README.md
+++ b/README.md
@@ -361,5 +361,11 @@
 sudo apt install wabt
 ```
 
+For use from Python, an unofficial PyPI package is available
+([`wabt-package`](https://github.com/natify-sh/wabt-package)):
+```sh
+pip install wabt-package
+```
+
 You can also download prebuilt binaries for many platforms directly from the
 github releases page.