Merge pull request #5 from swiftlang/shahmishal/doc-update

Update README.md to include details about the Swift package repo

Author: shahmishal

README.md

@@ -3,3 +3,34 @@
 This repository will contain reusable workflows to minimize redundant workflows across the organization. This effort will also facilitate the standardization of testing processes while empowering repository code owners to customize their testing plans as needed. The repository will contain workflows to support different types of repositories, such as Swift Package and Swift Compiler.
 
 For more details on reusable workflows, please refer to the [Reusing workflows](https://docs.github.com/en/actions/using-workflows/reusing-workflows) section in the GitHub Docs.
+
+## Reusable workflow for Swift package repositories
+
+To enable pull request testing for all supported Swift versions (5.8, 5.9, 5.10, 6.0, and nightly) on Linux, add the following code example in `.github/workflows/pull_request.yml`:
+
+```yaml
+name: pull_request
+
+on:
+  pull_request:
+    types: [opened, reopened, synchronize]
+
+jobs:
+  tests:
+    name: tests
+    uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@main
+```
+
+If your package only supports newer compiler versions, you can exclude older versions by using the `exclude_swift_versions` workflow input:
+
+```yaml
+exclude_swift_versions: "[{\"swift_version\": \"5.8\"}]"
+```
+
+Additionally, if your package requires additional installed packages, you can use the `pre_build_command`:
+
+```yaml
+pre_build_command: "apt-get update -y -q && apt-get install -y -q example"
+```
+
+macOS and Windows platform support will be available soon.