Adding mit-license badge to README.md (bayesiains#81)

Signed-off-by: Roman <rothr8@bfh.ch>

README.md

@@ -2,6 +2,7 @@
 
 <a href="https://doi.org/10.5281/zenodo.4296287"><img src="https://zenodo.org/badge/DOI/10.5281/zenodo.4296287.svg" alt="DOI"></a>
 <a href="https://github.com/bayesiains/nflows/actions/workflows/build_lint_test.yml"><img src="https://github.com/bayesiains/nflows/actions/workflows/build_lint_test.yml/badge.svg" alt="Build status"></a>
+<a href="https://opensource.org/licenses/MIT"><img src="https://img.shields.io/badge/License-MIT-green.svg"></a>
 
 `nflows` is a comprehensive collection of [normalizing flows](https://arxiv.org/abs/1912.02762) using [PyTorch](https://pytorch.org).