What to do with an expository note
(Upon jakebeal's request:)

Seconding @Kimball's recommendation, the "Monthly" (for all its problems...) is seen by undergrads, and also "College M.J.". Small consolation, but, indeed, it's hard to reach people who don't know the keyword to "google-upon". Conceivably, your enveloping HTML page could include more-naive keywords, or even nearly-hype-ish keywords, hubris-acknowledged, that would give g-hits.