CI: Add GitHub Actions

Author: rinsuki

.github/workflows/ci.yml

@@ -0,0 +1,21 @@
+name: CI
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+
+jobs:
+  build-and-test:
+    name: Build and Test
+    runs-on: macos-latest
+    strategy:
+      matrix:
+        xcode: # see https://github.com/actions/virtual-environments/tree/master/images/macos
+          - "11.4"
+          - "11.4.1"
+          - "11.5_beta"
+    steps:
+      - uses: actions/checkout@v2
+      - run: sudo xcode-select -s /Applications/Xcode_${{ matrix.xcode }}.app
+      - run: swift test

README.md

@@ -50,7 +50,6 @@ If you download and play with the sample app, it’s already cool. But if you re
 - [ ] Explain how to implement drag-n-drop in your app
 - [ ] Add versioning because now it’s just a `master` branch
 - [ ] Add this project to the [Dave’s](https://swiftpm.co) repository  of Swift Packages
-- [ ] Make a [GitHub Action]() for building and testing a Swift Package
 - [ ] Improve the Demo app, it’s too basic and doesn’t show all the features
 - [ ] File a Radar requesting a real `NSOutlineViewDiffableDataSource`, why not