Suggest using PoolAddressProvider instead of Pool

[klntsky]

Currently, the example code in readme looks like this:

const markets = require("@bgd-labs/aave-address-book");
...
    return new PoolBundle(provider, {
      POOL: markets[marketKey].POOL,
      WETH_GATEWAY: markets[marketKey].WETH_GATEWAY,
    });

This should not be the right way to do it, instead, the reader should be encouraged to fetch the pool address from the PoolAddressProvider contract.