Updated docs for zero protocol fees (#337)

docs/basics/protocol_fees.rst

@@ -3,10 +3,14 @@ Protocol Fees
 ###############################
 
 An ETH protocol fee is paid by the Taker each time a `Limit Order <./orders.html#limit-orders>`_ is `filled <./functions.html>`_.
-The fee is proportional to the gas cost of filling an order and scales linearly with gas price. The cost is currently ``70k * tx.gasprice``. 
+The fee is proportional to the gas cost of filling an order and scales linearly with gas price. The cost is currently ``0 * tx.gasprice``. 
 At the end of every Staking Epoch, these fees are aggregated and distributed to the makers as a liquidity reward: the reward is proportional to the maker's collected fees and staked ZRX relative to other makers.
 To learn more about protocol fees and liquidity incentives, see the `Official Spec <https://github.com/0xProject/0x-protocol-specification/blob/master/staking/staking-specification.md>`_.
 
+.. note::
+
+    As of September 29, 2021, protocol fees have been removed for all order types in both Exchange V4 and V3 in accordance with `ZEIP-91 <https://0x.org/zrx/vote/zeip-91>`_.
+
 .. note::
 
     `RFQ Orders <./orders.html#rfq-orders>`_ are introduced in Exchange V4, and there is currently no protocol fee for filling this type of order.