Add badge to README.md

[MatiasOlivera]

Fixes #263

[M-Scott-Lassiter]

Just curious why this hasn't been merged or closed yet... seems like a straightforward enough proposal. Can you help elaborate on what I'm missing when you said it needs design approval? If the logo is already on Simple Icons, and this repository is owned by Airbnb anyway, what is the legal/design problem?

[ljharb]

@M-Scott-Lassiter there’s no indication the existing badge was ever approved by Airbnb, for one; but also, Airbnb design must approve things like this every time. As soon as an Airbnb employee says it’s approved or rejected, this will be merged or closed - and until then, this can safely stay open for eternity.

[M-Scott-Lassiter]

@ljharb Interesting! Thanks for such quick feedback!