What do you call a note that gives preliminary information before the main part?

A suitable word for your brief introduction is preamble. It's not as formal as preface, and can be as short as a sentence (which would be unusual for a preface). Preamble can be countable or a mass noun, as Oxford Dictionaries shows:

1. A preliminary or preparatory statement; an introduction:
   he could tell that what she said was by way of a preamble
   I gave him the bad news without preamble

ODO

It's pronounced as three syllables, and does have the same root as amble — medieval Latin praeambulum, from late Latin praeambulus 'going before'.

NB: Maybe something might be interesting to note well.