Document how to import types like BigNumber

Importing base types like Contract or Wallet is trivial:

import { Contract, Wallet } from "ethers";

const myContract: Contract = ...

But I had to dig up the source code to figure out how to import BigNumber:

import { utils } from "ethers";

const myBigNumber: utils.BigNumber = ...

It’d be nice to have this explained in the docs.

[ricmoo]

I’ve been mulling back-and-forth how to add this to the new docs, especially since many things are available from their sub-modules too.

I’ll work on this on the next docs pass.

[ricmoo]

The package README.md files include this info, which in v6 will be linked to and will include a large import page that indicates where each class, function and interface comes from.