Brand guidelines don't mention favicons

[minrk]

This repo has favicon files (lacking moons due to small display size), but they aren't mentioned in the brand guidelines. This means they can't easily be used according to our trademark policy.

I think we should mention the moonless favicon files in the guidelines and suggest using them for "small sizes" e.g. favicons or very small icons (I don't know if we want to go beyond subjective "very small" and suggest a specific display size, e.g. below 24px square).