Warn on master (#45)

Add warning message when running on master

src/setup-dotnet.ts

@@ -8,6 +8,10 @@ async function run() {
     // Version is optional.  If supplied, install / use from the tool cache
     // If not supplied then task is still used to setup proxy, auth, etc...
     //
+    console.log(
+      `::warning::Use the v1 tag to get the last version, master may contain breaking changes and will not contain any required packages in the future. i.e. actions/setup-dotnet@v1`
+    );
+
     let version = core.getInput('version');
     if (!version) {
       version = core.getInput('dotnet-version');