Update README.md

Author: gares

README.md

@@ -1,5 +1,6 @@
 [![Docker CI](https://github.com/LPCIC/coq-elpi/actions/workflows/main.yml/badge.svg)](https://github.com/LPCIC/coq-elpi/actions/workflows/main.yml)
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+[![Nix CI](https://github.com/LPCIC/coq-elpi/actions/workflows/nix-action-coq-8.19.yml/badge.svg)](https://github.com/LPCIC/coq-elpi/actions/workflows/nix-action-coq-master.yml)
 [![DOC](https://github.com/LPCIC/coq-elpi/actions/workflows/doc.yml/badge.svg)](https://github.com/LPCIC/coq-elpi/actions/workflows/doc.yml)
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 <img align="right" src="https://github.com/LPCIC/coq-elpi/raw/master/etc/logo.png" alt="Coq-Elpi logo" width="25%" />