Add GitHub Actions badge (#408)

Author: tkf

README.md

@@ -5,6 +5,7 @@ PyJulia
 [![Latest documentation](https://img.shields.io/badge/docs-latest-blue.svg)](https://pyjulia.readthedocs.io/en/latest/)
 [![Build Status](https://travis-ci.org/JuliaPy/pyjulia.svg?branch=master)](https://travis-ci.org/JuliaPy/pyjulia)
 [![Build status](https://ci.appveyor.com/api/projects/status/github/JuliaPy/pyjulia?svg=true)](https://ci.appveyor.com/project/Keno/pyjulia)
+[![Main workflow](https://github.com/JuliaPy/pyjulia/workflows/Main%20workflow/badge.svg)](https://github.com/JuliaPy/pyjulia/actions?query=workflow%3A%22Main+workflow%22)
 
 Experimenting with developing a better interface to [Julia language](https://julialang.org/) that works with [Python](https://www.python.org/) 2 & 3 and Julia v1.0+.