Revert "Remove CircleCI support"

This reverts commit fd6ce8c.

.circleci/config.yml

@@ -0,0 +1,24 @@
+defaults: &defaults
+    macos:
+        xcode: 12.4.0
+
+version: 2.1
+
+jobs:
+    Build and Test:
+        <<: *defaults
+        steps:
+        - checkout
+        - run:
+            name: Build Tests
+            command: swift build --build-tests --disable-automatic-resolution -v
+        - run:
+            name: Run Tests
+            command: swift test --disable-automatic-resolution -v
+
+workflows:
+    version: 2
+
+    Build and Test:
+        jobs:
+            - Build and Test

README.md

@@ -4,6 +4,8 @@ Heimdallr is an [OAuth 2.0](https://tools.ietf.org/html/rfc6749) client specific
 
 If you are an Android Developer, please take a look at the [Android version of Heimdallr](https://github.com/trivago/Heimdall.droid).
 
+[![Build Status](https://circleci.com/gh/trivago/Heimdallr.swift.svg?style=shield&circle-token=06d0c39133fae3dd9b649c116776c7f882885f1f)](https://circleci.com/gh/trivago/Heimdallr)
+
 [![Build Status](https://github.com/trivago/Heimdallr.swift/actions/workflows/heimdallr.yml/badge.svg)](https://github.com/trivago/Heimdallr.swift/actions/workflows/heimdallr.yml)
 
 ## Example