Update document to notify that `binance-smart-chain` is now supported with token contracts API

[phatNfqAsia]

As I read here https://github.com/miscavage/CoinGecko-API/blob/master/README.md#coinsfetchcoincontractinfo, it says currently unsupported with BSC but when I call

const response = await client.contractMarketChartRange({
    id: "binance-smart-chain" as any,
    contract_address: cakeTokenCotractAddress,
    vs_currency: "usd",
    from: 1647101750,
    to: 1647131756,
  });

it gave response correctly so I suggest update documentatiot for avoid unnecessary guessing effort from users. Just confirm that you are still standing by to review and merge PR, I will do it for you guys.