Baseline defintion: flag undefined status situations

[ddbeck]

#2133 prompted me to add two non-normative notes to the Baseline definition, explicitly highlighting some (IMO, intentionally) undefined qualities of the definition.

[ddbeck]

If we wanted to, we could make some statement about what we expect to happen if we were to add a browser and that browser did not implement the full Baseline feature set at the time the browser was added. We could do that as part of this PR or in a later one.

Some ideas for that, from strongest to weakest:

• Freeze the browser set for those features (i.e., only judge existing Baseline low/high features on the "old" browser set)
• Declare some minimum transition period (e.g., additions to the core browser set take 30 months to negatively effect existing Baseline features)
• Offer a warning period (e.g., we won't change the core browser set in a way that would remove features from the Baseline feature set without giving at least 3 months notice)

[captainbrosset]

Thanks for adding this, Daniel. At least adding some notes about how the situation is not yet defined is useful.