document global-json-file

README.md

@@ -53,6 +53,16 @@ steps:
     include-prerelease: true
 - run: dotnet build <my project>
 ```
+global.json in a subdirectory:
+```yml
+steps:
+- uses: actions/checkout@v2
+- uses: actions/setup-dotnet@v1
+  with:
+    global-json-file: csharp/global.json
+- run: dotnet build <my project>
+  working-directory: csharp
+```
 
 Matrix Testing:
 ```yaml

src/setup-dotnet.ts

@@ -9,7 +9,9 @@ export async function run() {
     //
     // dotnet-version is optional, but needs to be provided for most use cases.
     // If supplied, install / use from the tool cache.
-    // If not supplied, look for version in ./global.json.
+    // global-version-file may be specified to point to a specific global.json
+    // and will be used to install an additional version.
+    // If neither supplied, look for version in ./global.json.
     // If a valid version still can't be identified, nothing will be installed.
     // Proxy, auth, (etc) are still set up, even if no version is identified
     //