Update README.md

Author: albertodebortoli

README.md

@@ -1,2 +1,85 @@
 # select-xcode
-GitHub action to select Xcode via 1. explicit input 2. .xcode-version file 3. env var
+
+A GitHub Actions composite action that selects a specific Xcode version on macOS runners via `xcode-select`.
+
+## Version resolution order
+
+The action resolves the Xcode version using the following priority:
+
+1. **`xcode-version` input** — explicit version passed to the action
+2. **`.xcode-version` file** — a file at the root of the repository containing the version string
+3. **`XCODE_VERSION` environment variable** — a variable already set in the workflow environment
+
+If none of the above provides a version, the action fails with a descriptive error message.
+
+## Usage
+
+### With an explicit version input
+
+```yaml
+- uses: LucaTools/select-xcode@main
+  with:
+    xcode-version: "26.2"
+```
+
+### With a `.xcode-version` file
+
+Commit a `.xcode-version` file to the root of your repository:
+
+```
+26.2
+```
+
+Then use the action without any input:
+
+```yaml
+- uses: LucaTools/select-xcode@main
+```
+
+### With a pre-set environment variable
+
+```yaml
+env:
+  XCODE_VERSION: "26.2"
+
+steps:
+  - uses: LucaTools/select-xcode@main
+```
+
+## Inputs
+
+| Name            | Required | Default | Description                     |
+|-----------------|----------|---------|---------------------------------|
+| `xcode-version` | No       | `""`    | The version of Xcode to select. |
+
+## Full workflow example
+
+```yaml
+name: CI
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  build:
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: LucaTools/select-xcode@main
+        with:
+          xcode-version: "26.2"
+
+      - name: Build
+        run: xcodebuild build -scheme MyApp
+```
+
+## Requirements
+
+- Must run on a macOS runner (e.g. `macos-latest`, `macos-26`).
+- The requested Xcode version must be available at `/Applications/Xcode_<version>.app`.
+
+## License
+
+[MIT](LICENSE)