Add a badge for the regular conformance tests.

Author: thomasvl

README.md

@@ -18,6 +18,7 @@ files, you will need to add this library to your project.
 
 [![Build and Test](https://github.com/apple/swift-protobuf/workflows/Build%20and%20Test/badge.svg)](https://github.com/apple/swift-protobuf/actions?query=workflow%3A%22Build+and+Test%22)
 [![Check Upstream Protos](https://github.com/apple/swift-protobuf/workflows/Check%20Upstream%20Proto%20Files/badge.svg)](https://github.com/apple/swift-protobuf/actions?query=workflow%3A%22Check+Upstream+Proto+Files%22)
+[![Run Conformance Tests](https://github.com/apple/swift-protobuf/workflows/Run%20Conformance%20Tests/badge.svg)](https://github.com/apple/swift-protobuf/actions?query=workflow%3A%22Run=Conformance+Tests%22)
 
 # Features of SwiftProtobuf