Add license badge

README.md

@@ -14,7 +14,7 @@ exo: Run your own AI cluster at home with everyday devices. Maintained by [exo l
 
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-[![GitHub Repo stars](https://img.shields.io/github/stars/exo-explore/exo)](https://github.com/exo-explore/exo/stargazers)
+[![GitHub Repo stars](https://img.shields.io/github/stars/exo-explore/exo)](https://github.com/exo-explore/exo/stargazers) [![License: GPL v3](https://img.shields.io/badge/License-GPLv3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)
 
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