Support notes

[foolip]

As suggested by @captainbrosset in #546, we'll need a notes field similar to BCD at some point.

The most straightforward is to use BCD-style support statements, but I think mdn/browser-compat-data#17857 is important. If we don't know which versions a note applies to, consumers of web-features won't know when it's safe to stop showing it.

[captainbrosset]

This was discussed in today's WebDX CG Baseline call. Link to notes: https://docs.google.com/document/d/1ree75ImLZjf60lTZ3BhCaLHygxgywr7SBXp-q0xPs8A/edit#heading=h.jzil2mv8ygpy

@ddbeck will likely do an initial proposal in #503.