add status badge (#10)

Author: frapac

README.md

@@ -1,5 +1,7 @@
 # COPSBenchmark.jl
 
+[![Run tests](https://github.com/MadNLP/COPSBenchmark.jl/actions/workflows/action.yml/badge.svg)](https://github.com/MadNLP/COPSBenchmark.jl/actions/workflows/action.yml)
+
 This package implements using JuMP a subset of the [COPS benchmark](https://www.mcs.anl.gov/~more/cops/),
 originally implemented in AMPL.