Message box if checkbox is checked when hitting "Submit" button

Hi,

I'm building a form that has checkboxes for "Yes" and "No" and a submit button that sends the form to an email address.  This is what I want to happen:

Click the submit button.

If the "Yes" checkbox is selected, a popup message will say, "Please attach supporting documents to the email."  Otherwise, it just generates the email.

The name of the checkbox is "Yes" and the name of the button is "Submit".  I'm new to js, and I haven't been able to find the coding (or figure it out enough to modify it, I guess) to do this.