Add description & git URI fields

field name: description

A human readable short paragraph that describes the chain.

type: long text. No limit, but suggested to keep it tweet length, 280 characters. May contain Markdown.

field name: git

A link to a git repo or organization that represents the chain.

type: URI

This _could_ be an array, maybe keep it simple and just be empty or 1 URI.

Ethereum

description: The main public chain for Ethereum
git: https://github.com/ethereum

Ethereum Classic

description: The Classic Ethereum chain that resulted from the split after the DAO hack.
git: https://github.com/ethereumclassic

Görli

description: The Görli testnet
git: https://github.com/goerli

I started with git as we can focus on technical resources. A website field could also be added, but then I would have to link to EthereumDotOrg and be embarrassed 🤣