docs: added Zenodo rolling release badge

README.md

@@ -13,6 +13,7 @@
 ![Codecov](https://img.shields.io/codecov/c/github/fortran-lang/fortls?style=flat-square)
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg?style=flat-square)](https://github.com/psf/black)
 ![GitHub Repo stars](https://img.shields.io/github/stars/fortran-lang/fortls?color=yellow&style=flat-square)
+[![DOI](https://zenodo.org/badge/412392321.svg?style=flat-square)](https://zenodo.org/badge/latestdoi/412392321)
 
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