Enhance README with citation and DOI badge

Add citation information and a DOI badge to README.

Author: ss0832

README.md

@@ -10,6 +10,8 @@
 [![Buy Me A Coffee](https://img.shields.io/badge/Buy%20Me%20a%20Coffee-ffdd00?style=for-the-badge&logo=buy-me-a-coffee&logoColor=black)](https://buymeacoffee.com/ss0832)
 
 [![PyPI Downloads](https://static.pepy.tech/personalized-badge/multioptpy?period=total&units=INTERNATIONAL_SYSTEM&left_color=BLACK&right_color=GREEN&left_text=downloads)](https://pepy.tech/projects/multioptpy)
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.17839100.svg)](https://doi.org/10.5281/zenodo.17839100)
+
 
 If this tool helped your studies, education, or saved your time, I'd appreciate a coffee!
 Your support serves as a great encouragement for this personal project and fuels my next journey.
@@ -346,8 +348,23 @@ highlighty876[at]gmail.com
 
 ## Citation
 
-ss0832. (2025). MultiOptPy: Multifunctional geometry optimization tools for quantum chemical calculations (v1.20.2). Zenodo. https://doi.org/10.5281/zenodo.17839100
+If you use MultiOptPy in your research, please cite it as follows:
 
+```bibtex
+@software{ss0832_multioptpy_2025,
+  author       = {ss0832},
+  title        = {MultiOptPy: Multifunctional geometry optimization tools for quantum chemical calculations},
+  month        = dec,
+  year         = 2025,
+  publisher    = {Zenodo},
+  version      = {v1.20.2},
+  doi          = {10.5281/zenodo.17839100},
+  url          = {[https://doi.org/10.5281/zenodo.17839100](https://doi.org/10.5281/zenodo.17839100)}
+}
+```
+```
+ss0832. (2025). MultiOptPy: Multifunctional geometry optimization tools for quantum chemical calculations (v1.20.2). Zenodo. https://doi.org/10.5281/zenodo.17839100
+```
 
 ## Setting Up an Environment for Using NNP(UMA) on Windows 11