Give context for dues suggestions

[brittag]

What github issue is this PR for, if any?

None

What does this code do, and why?

On the dues setup page and manage dues page, I added information about DU's average monthly expenses and a link to our mini budget sheet.

I helped a person go through membership setup today, and they found the suggested dues table a bit surprising. We discussed the context (DU's expenses and income), which was helpful. So I figure this information may be helpful for other new members as well.

How is this code tested?

It's not

Are any database migrations required by this change?

No

Are there any configuration or environment changes needed?

No

Screenshots please :)

[brittag]

The linked spreadsheet is now outdated, so this shouldn't be merged as-is.