go mod tidy will remove the reference to github.com/dmarkham/enumer from go.mod

[madkins23]

This can be prevented by adding a tools.go file:

//go:build tools

package tools

import (
	// Protect this entry in go.mod from being removed by go mod tidy.
	_ "github.com/dmarkham/enumer"
)

Should probably go into README.md.