Explain that `__init__` is needed when using Import Linter

**Motivation**
As per #128, we will enable `INP` when the user adds Import Linter while using Ruff.

But if they're not using Ruff, providing some explicit guidance in a message would be helpful.

**Summary of desired enhancement**
Explain that `__init__` is needed when using Import Linter in the `ImportLinterTool.print_how_to_use` method when not using Ruff.
