Add a canary file to suggest Go 1.10 is required

Closes #22

go110min.go

@@ -0,0 +1,8 @@
+//+build !go1.10
+
+package main
+
+// This file is here to give a better hint in the error message
+// when this project is built with a too old version of Go.
+
+var _ = ThisProjectRequiresGo1·10OrHigher