Add upstream readme badge

Author: ravwojdyla

README.md

@@ -3,6 +3,7 @@
 [![Windows build status](https://github.com/pystatgen/sgkit/workflows/Windows/badge.svg?branch=master)](https://github.com/pystatgen/sgkit/actions?query=workflow%3A%22Windows%22+branch%3Amaster)
 [![Documentation status](https://github.com/pystatgen/sgkit/workflows/Docs/badge.svg?branch=master)](https://pystatgen.github.io/sgkit/)
 [![Validation status](https://github.com/pystatgen/sgkit/workflows/Validation/badge.svg?branch=master)](https://github.com/pystatgen/sgkit/actions?query=workflow%3A%22Validation%22+branch%3Amaster)
+[![Upstream status](https://github.com/pystatgen/sgkit/workflows/Upstream/badge.svg?branch=master)](https://github.com/pystatgen/sgkit/actions?query=workflow%3A%22Upstream%22+branch%3Amaster)
 
 Statistical genetics toolkit