Add a simple build workflow

.github/workflows/swift.yml

@@ -0,0 +1,28 @@
+name: Build and test
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  macos_build:
+
+    runs-on: macos-10.15
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Build
+      run: swift build -v
+
+  linux_build:
+
+    runs-on: ubuntu-18.04
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Test on Ubuntu with Swift 5.2
+      uses: Didstopia/SwiftAction@v1.0.2
+      with:
+        swift-action: build