Add stub for shell

shell.go

@@ -0,0 +1,15 @@
+package adb
+
+import (
+	"context"
+)
+
+// Shell allows you to run an arbitrary shell command against a device.
+//
+// This function is useful if you need to run an obscure shell command or if
+// you require functionality not provided by the exposed functions here.
+// Instead of using Shell, please consider submitting a PR with the functionality
+// you require.
+func (d Device) Shell(ctx context.Context, command string) (stdout string, stderr string, ErrCode int, err error) {
+	return "", "", 1, nil
+}