Add new CI badge to README.

Author: arthuraa

README.md

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 # Deriving ─ Generic instances for Coq inductive types
 
+[![Nix Flake
+actions](https://github.com/arthuraa/deriving/actions/workflows/cachix-install-nix-action.yml/badge.svg)](https://github.com/arthuraa/deriving/actions/workflows/cachix-install-nix-action.yml)
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 The Deriving library builds instances of basic MathComp classes for inductive
 data types with little boilerplate, akin to Haskell's `deriving` functionality.
 To define an `eqType` instance for a type `foo`, just write: