add citation to readme

CITATION.cff

@@ -0,0 +1,19 @@
+cff-version: 1.2.0
+message: "If you use pyfixest in your research, please cite it as below."
+title: "pyfixest: Fast high-dimensional fixed effect estimation in Python"
+type: software
+authors:
+  - name: "The PyFixest Authors"
+
+repository-code: 'https://github.com/py-econometrics/pyfixest'
+url: 'https://py-econometrics.github.io/pyfixest/pyfixest.html'
+abstract: "A Python package for fast high-dimensional fixed effect estimation, inspired by the syntax of the popular R package `fixest`. It provides a familiar and intuitive interface for economists and other social scientists to estimate complex regression models with multiple fixed effects efficiently."
+keywords:
+  - 'fixed effects'
+  - 'panel data'
+  - 'econometrics'
+  - 'statistics'
+  - 'causal inference'
+  - 'regression'
+  - 'python'
+license: MIT

README.md

@@ -15,6 +15,9 @@
 [![Ruff](https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/astral-sh/ruff/main/assets/badge/v2.json)](https://github.com/astral-sh/ruff)
 [![Pixi Badge][pixi-badge]][pixi-url]
 [![Donate | GiveDirectly](https://img.shields.io/static/v1?label=GiveDirectly&message=Donate&color=blue&style=flat-square)](https://github.com/py-econometrics/pyfixest?tab=readme-ov-file#support-pyfixest)
+[![PyPI](https://img.shields.io/pypi/v/pyfixest)](https://pypi.org/project/pyfixest)
+[![PyPI - Python Version](https://img.shields.io/pypi/pyversions/pyfixest)](https://pypi.org/project/pyfixest/)
+[![Citation](https://img.shields.io/badge/Cite%20as-PyFixest-blue)](https://github.com/py-econometrics/pyfixest?tab=readme-ov-file#how-to-cite)
 
 [pixi-badge]:https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/prefix-dev/pixi/main/assets/badge/v0.json&style=flat-square
 [pixi-url]: https://pixi.sh
@@ -306,3 +309,16 @@ This project follows the [all-contributors](https://github.com/all-contributors/
 We thank all institutions that have funded or supported work on PyFixest!
 
 <img src="figures/aai-institute-logo.svg" width="185">
+
+## How to Cite
+
+If you want to cite PyFixest, you can use the following BibTeX entry:
+
+```bibtex
+@software{pyfixest,
+  author  = {{The PyFixest Authors}},
+  title   = {{pyfixest: Fast high-dimensional fixed effect estimation in Python}},
+  year    = {2025},
+  url     = {https://github.com/py-econometrics/pyfixest}
+}
+```

docs/pyfixest.md

@@ -14,6 +14,7 @@
 [![Downloads](https://static.pepy.tech/badge/pyfixest/month)](https://pepy.tech/project/pyfixest)
 [![Ruff](https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/astral-sh/ruff/main/assets/badge/v2.json)](https://github.com/astral-sh/ruff)
 [![Pixi Badge][pixi-badge]][pixi-url]
+[![Citation](https://img.shields.io/badge/Cite%20as-PyFixest-blue)](https://github.com/py-econometrics/pyfixest?tab=readme-ov-file#how-to-cite)
 [![Donate | GiveDirectly](https://img.shields.io/static/v1?label=GiveDirectly&message=Donate&color=blue&style=flat-square)](https://github.com/py-econometrics/pyfixest?tab=readme-ov-file#support-pyfixest)
 
 [pixi-badge]:https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/prefix-dev/pixi/main/assets/badge/v0.json&style=flat-square
@@ -257,3 +258,17 @@ This project follows the [all-contributors](https://github.com/all-contributors/
 We thank all institutions that have funded or supported work on PyFixest!
 
 <img src="../figures/aai-institute-logo.svg" width="185">
+
+
+## How to Cite
+
+If you want to cite PyFixest, you can use the following BibTeX entry:
+
+```bibtex
+@software{pyfixest,
+  author  = {{The PyFixest Authors}},
+  title   = {{pyfixest: Fast high-dimensional fixed effect estimation in Python}},
+  year    = {2025},
+  url     = {https://github.com/py-econometrics/pyfixest}
+}
+```