docs: document automatic go.mod detection

README.md

@@ -187,11 +187,25 @@ from thils file will be:
 
 The version can specify a patch version or omit it altogether (e.g., `go 1.22.0` or `go 1.22`).
 
-If a patch version is specified, that specific patch version will be used.  
+If a patch version is specified, that specific patch version will be used.
 If no patch version is specified, it will search for the latest available patch version in the cache,
 [versions-manifest.json](https://github.com/actions/go-versions/blob/main/versions-manifest.json), and the
 [official Go language website](https://golang.org/dl/?mode=json&include=all), in that order.
 
+### Automatic go.mod detection
+
+**New in v6**: If neither `go-version` nor `go-version-file` is specified, the action will automatically look for a `go.mod` file in the repository root and use the Go version specified in it. This simplifies workflows for projects that already have a `go.mod` file.
+
+```yaml
+steps:
+  - uses: actions/checkout@v5
+  - uses: actions/setup-go@v6
+    # Automatically uses go.mod from repository root
+  - run: go version
+```
+
+### Specifying a custom path
+
 If both the `go-version` and the `go-version-file` inputs are provided then the `go-version` input is used.
 > The action will search for the `go.mod` file relative to the repository root
 

action.yml

@@ -5,7 +5,7 @@ inputs:
   go-version:
     description: 'The Go version to download (if necessary) and use. Supports semver spec and ranges. Be sure to enclose this option in single quotation marks.'
   go-version-file:
-    description: 'Path to the go.mod or go.work file.'
+    description: 'Path to the go.mod or go.work file. If not specified, the action will automatically use go.mod from the workspace root if it exists.'
   check-latest:
     description: 'Set this option to true if you want the action to always check for the latest available version that satisfies the version spec'
     default: false