Docs: Add NAPAC'22 Paper (#206)

Add our latest NAPAC'22 paper to the documented references.
Will replace with a JACOW reference once proceedings are out.

README.md

@@ -3,7 +3,8 @@
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-[![DOI (source)](https://img.shields.io/badge/DOI%20(source)-10.5281/zenodo.6954922-blue.svg)](https://doi.org/10.5281/zenodo.6954922)  
+[![DOI (source)](https://img.shields.io/badge/DOI%20(source)-10.5281/zenodo.6954922-blue.svg)](https://doi.org/10.5281/zenodo.6954922)
+[![DOI (paper)](https://img.shields.io/badge/DOI%20(paper)-10.48550/arXiv.2208.02382-blue.svg)](https://doi.org/10.48550/arXiv.2208.02382)  
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docs/source/acknowledge_us.rst

@@ -31,6 +31,9 @@ Please add the following sentence to your publications, it helps contributors ke
 Main ImpactX reference
 **********************
 
-.. note::
+If your project leads to a scientific publication, please consider citing the paper below.
 
-   TODO :-)
+- Huebl A, Lehe R, Mitchell C E, Qiang J, Ryne R D, Sandberg R T, Vay JL.
+  **Next Generation Computational Tools for the Modeling and Design of Particle Accelerators at Exascale**.
+  *NAPAC'22*, 2022.
+  `arXiv:2208.02382 <https://arxiv.org/abs/2208.02382>`__