add linux file that will give helpful compiler error

certstore_linux.go

@@ -0,0 +1,14 @@
+package certstore
+
+import "errors"
+
+// This will hopefully give a compiler error that will hint at the fact that
+// this package isn't designed to work on Linux.
+func init() {
+	CERTSTORE_DOESNT_WORK_ON_LINIX
+}
+
+// Implement this function, just to silence other compiler errors.
+func openStore() (Store, error) {
+	return nil, errors.New("certstore only works on macOS and Windows")
+}