Update README.md

README.md

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 Note that GPU-based implementations are NOT included in this package.
 
-If you find this package useful for your project, please consider leaving a comment [here](https://github.com/koide3/small_gicp/issues/3). It would help the author receive recognition in his organization and keep working on this project.
+If you find this package useful for your project, please consider leaving a comment [here](https://github.com/koide3/small_gicp/issues/3). It would help the author receive recognition in his organization and keep working on this project. Please also cite [the corresponding software paper](https://joss.theoj.org/papers/10.21105/joss.06948) if you use this software in an academic work.
 
 
 [![status](https://joss.theoj.org/papers/059b017c823ed9fd211fc91373cdc2cc/status.svg)](https://joss.theoj.org/papers/059b017c823ed9fd211fc91373cdc2cc) [![Build(Linux)](https://github.com/koide3/small_gicp/actions/workflows/build-linux.yml/badge.svg)](https://github.com/koide3/small_gicp/actions/workflows/build-linux.yml) [![macos](https://github.com/koide3/small_gicp/actions/workflows/build-macos.yml/badge.svg)](https://github.com/koide3/small_gicp/actions/workflows/build-macos.yml) [![Build(Windows)](https://github.com/koide3/small_gicp/actions/workflows/build-windows.yml/badge.svg)](https://github.com/koide3/small_gicp/actions/workflows/build-windows.yml) [![Test](https://github.com/koide3/small_gicp/actions/workflows/test.yml/badge.svg)](https://github.com/koide3/small_gicp/actions/workflows/test.yml) [![codecov](https://codecov.io/gh/koide3/small_gicp/graph/badge.svg?token=PCVIUP2Z33)](https://codecov.io/gh/koide3/small_gicp)