include a placeholder main.go and enable go modules

Author: neolit123

go.mod

@@ -0,0 +1,3 @@
+module sigs.k8s.io/cluster-api-bootstrap-provider-kubeadm
+
+go 1.12

go.sum

@@ -0,0 +1,27 @@
+// placeholder file
+
+/*
+Copyright 2019 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	fmt.Println("hello world")
+}