📝 add readme

Author: ltetzlaff

readme.md

@@ -0,0 +1,33 @@
+# swift-api-diff
+
+This generates per-PR diff reports in the public API of a swift module. It's based on the [Swift-NIO team's api breakage script](https://github.com/apple/swift-nio/blob/master/scripts/check_no_api_breakages.sh)
+
+## [Usage example](https://github.com/ltetzlaff/swift-api-diff-example/blob/master/.github/workflows/swift-api.yml)
+
+```yml
+name: SwiftAPIBreakage
+
+on:
+  pull_request:
+    types: [opened, synchronize]
+
+jobs:
+  buildAndCheckAPIBreakage:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          ref: master
+          fetch-depth: 0
+      - uses: ltetzlaff/swift-api-diff@master
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+```
+
+### Example with a change
+
+https://github.com/ltetzlaff/swift-api-diff-example/pull/4
+
+### Example without a change
+
+https://github.com/ltetzlaff/swift-api-diff-example/pull/3