docs: Add Zenodo DOI to CITATION.cff and README, and update citation instructions.

Author: skilledwolf

CITATION.cff

@@ -3,6 +3,7 @@ message: "If you use this software, please cite it as below."
 type: software
 title: "quantumhall_matrixelements: Quantum Hall Landau-Level Matrix Elements"
 version: "0.1.0"
+doi: 10.5281/zenodo.17646027
 date-released: "2025-11-18"
 authors:
   - family-names: Wolf
@@ -12,4 +13,3 @@ license: MIT
 abstract: >
   Landau-level plane-wave form factors and exchange kernels for quantum Hall systems,
   with multiple numerical backends and symmetry diagnostics.
-

README.md

@@ -1,5 +1,7 @@
 # quantumhall-matrixelements: Quantum Hall Landau-Level Matrix Elements
 
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.17646027.svg)](https://doi.org/10.5281/zenodo.17646027)
+
 Landau-level plane-wave form factors and exchange kernels for quantum Hall systems.
 
 This library factors out the continuum matrix-element kernels used in Hartree–Fock and
@@ -113,9 +115,12 @@ For more detailed examples, see the example scripts under `examples/` and the te
 
 ## Citation
 
-If you use the package`quantumhall-matrixelements` in academic work, please cite:
+If you use the package `quantumhall-matrixelements` in academic work, please cite:
+
+> Tobias Wolf, *quantumhall-matrixelements: Quantum Hall Landau-Level Matrix Elements*, version 0.1.0, 2025.  
+> DOI: https://doi.org/10.5281/zenodo.17646027
 
-> Tobias Wolf, *quantumhall-matrixelements: Quantum Hall Landau-Level Matrix Elements*, version 0.1.0, 2025. DOI: to be assigned via Zenodo.
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.17646027.svg)](https://doi.org/10.5281/zenodo.17646027)
 
 A machine-readable `CITATION.cff` file is included in the repository and can be used with tools that support it (for example, GitHub’s “Cite this repository” button).