Add citation preference in readme

README.md

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 [![codecov](https://codecov.io/gh/lsst/daf_butler/branch/main/graph/badge.svg?token=2BUBL8R9RH)](https://codecov.io/gh/lsst/daf_butler)
 
 LSST Data Access framework described in [arXiv:2206.14941](https://arxiv.org/abs/2206.14941).
+Please cite [the SPIE paper](https://doi.org/10.1117/12.2629569) when using this software.
 
 This is a **Python 3 only** package (we assume Python 3.10 or higher).
 
-* SPIE Paper from 2022: [The Vera C. Rubin Observatory Data Butler and Pipeline Execution System](https://arxiv.org/abs/2206.14941)
+* SPIE Paper from 2022: [The Vera C. Rubin Observatory Data Butler and Pipeline Execution System](https://doi.org/10.1117/12.2629569) ([arXiv](https://arxiv.org/abs/2206.14941))
 * ADASS paper from 2019: [Abstracting the Storage and Retrieval of Image Data at the LSST](https://ui.adsabs.harvard.edu/abs/2019ASPC..523..653J/abstract).
 * Early design note: [DMTN-056](https://dmtn-056.lsst.io)