doc: add config for welcome bot

This pull request adds configuration for https://github.com/behaviorbot/welcome, a bot that comments on issues and pull requests from first-time contributors. This was discussed during this week's Node.js Collaborator Summit as a way of making the Node.js project more welcoming to new contributors.

When and if this configuration lands in the repository, we'll need to actually install the bot. Maybe @trott can help with that.

Special thanks to @hiimbex for creating this bot 🙏

// @Bnb @hackygolucky @obensource @Tiriel

• documentation is changed or added
• commit message follows commit guidelines