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

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`.