Update README.md (#16)

Author: makslevental

README.md

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 # Minimal MLIR
 
+[![Nightly Test](https://github.com/makslevental/mmlir/actions/workflows/test.yml/badge.svg)](https://github.com/makslevental/mmlir/actions/workflows/test.yml)
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 This repo demos a slightly unconventional way to bootstrap an MLIR project:
 
 1. It relies on https://makslevental.github.io/wheels for the upstream distribution of MLIR