Update NOTES guidance in CONTRIBUTING

[jseldess]

After #1014, it's now possible to use markdown formatting within notes, but this only works if the liquid tags before and after the note content are on their own line. Need to update CONTRIBUTING.md accordingly.

[jseldess]

Turns out that adding the ability to use markdown inside alerts messes up how alerts get rendered (closing div doesn't work properly). I've reverted that change here: #1020. Closing this since the CONTRIBUTING.md should continue to encourage use of HTML inside alerts.