Update https://docs.radicle.xyz/ to include information about Radicle token

I think it makes sense to add information about the RAD token to our gitbook.
— Rant:
Frankly, I feel RAD token is being treated harshly. No major exchanges listing. no telegram or discord to engage crypto traders and investors.
I understand the need to protect RAD from pump-and-dumpers, but getting isolated from a major part of the crypto ecosystem [traders and speculators] is not the right way.
— End of rant

Awesome! I agree. Maybe you want to make an issue in https://github.com/radicle-dev/radicle-docs/issues/new ?