Add guide for EIP-712 signatures with vm.sign / vm.addr

[gakonst]

Explain how EIP712 works and create & verify signatures via the sign & addr cheatcodes https://github.com/Rari-Capital/solmate/blob/7c34ed021cfeeefb1a4bff7e511a25ce8a68806b/src/test/ERC20.t.sol#L89-L103

[tbtstl]

@onbjerg Are you actively working on this? Happy to dig in now if you'd like, we're actively working on this for a ZORA PR

[onbjerg]

@tbtstl Feel free to take it on! I just assigned myself to a bunch of issues I'm going to address soon, but the more the merrier 😄

[onbjerg]

Hey @tbtstl, just pinging to hear if you're still working on this 😊

[kulkarohan]

hey @onbjerg, yup just making some final changes and i'll push!