Add Poetry badge

New in Poetry 1.6:
https://python-poetry.org/blog/announcing-poetry-1.6.0/#official-poetry-badge
https://python-poetry.org/docs/community/#badge

README.md

@@ -3,6 +3,7 @@
 
 [![Tests](https://github.com/jond01/xil/actions/workflows/tests.yml/badge.svg)](https://github.com/jond01/xil/actions/workflows/tests.yml)
 [![pre-commit](https://img.shields.io/badge/pre--commit-enabled-brightgreen?logo=pre-commit)](https://github.com/pre-commit/pre-commit)
+[![Poetry](https://img.shields.io/endpoint?url=https://python-poetry.org/badge/v0.json)](https://python-poetry.org/)
 [![Ruff](https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/charliermarsh/ruff/main/assets/badge/v2.json)](https://github.com/charliermarsh/ruff)
 [![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/)