docs: update README with arXiv preprint citation for v0.2.0 paper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: mbuze

README.md

@@ -58,11 +58,11 @@ The examples from the paper can be found in [`notebooks/paper_examples/`](notebo
 
 </details>
 
-### (2) [forthcoming]
+### (2) Preprint:
 
-v0.2.0 accompanies a forthcoming paper on polynomial diagrams (generalisations of APDs):
+v0.2.0 accompanies a preprint on polynomial diagrams (generalisations of APDs):
 
-M. Buze et al., Polynomial diagrams for polycrystal modelling, *in preparation* (2026).
+D.P. Bourne, M. Buze, T. Gallouët, Q. Mérigot, Polynomial diagrams for microstructure modelling, arXiv:2605.20816 (2026). [arXiv](https://arxiv.org/abs/2605.20816)
 
 The companion notebooks (EBSD benchmark on Tata Steel data, synthetic APD reconstruction) can be found in [`notebooks/paper_examples/pmd_paper/`](notebooks/paper_examples/pmd_paper/):
 - Large EBSD benchmark [![nbviewer](https://raw.githubusercontent.com/jupyter/design/master/logos/Badges/nbviewer_badge.svg)](https://nbviewer.org/github/mbuze/PyAPD/blob/main/notebooks/paper_examples/pmd_paper/big_ebsd.ipynb?flush_cache=true) [![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/mbuze/PyAPD/blob/main/notebooks/paper_examples/pmd_paper/big_ebsd.ipynb)
@@ -91,14 +91,14 @@ keywords = {Anisotropic power diagrams, Polycrystalline materials, Microstructur
 
 If you use the polynomial diagram functionality (`min_diagram_system`) or the EBSD interoperability features introduced in v0.2.0, please also cite:
 ```
-@article{PyAPD_PMD,
-  author  = {M. Buze and others},
-  title   = {Polynomial diagrams for polycrystal modelling},
-  journal = {in preparation},
-  year    = {2026},
+@misc{PyAPD_PMD,
+  author        = {David P. Bourne and Maciej Buze and Thomas Gallouët and Quentin Mérigot},
+  title         = {Polynomial diagrams for microstructure modelling},
+  year          = {2026},
+  eprint        = {2605.20816},
+  archivePrefix = {arXiv},
 }
 ```
-*(This entry will be updated with full bibliographic details upon publication.)*
 
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