Add guide for how to set token allowances

Author: jlin27

mdx/api/index.mdx

@@ -68,6 +68,8 @@ Some interactions with 0x require or are improved by setting [token allowances](
 * Paying protocol fees in WETH, you will need to give a WETH allowance to the [StakingProxy](https://0x.org/docs/guides/v3-staking-specification#architecture)
 * **Note:** For swaps with "ETH" as sellToken, wrapping "ETH" to "WETH" or unwrapping "WETH" to "ETH" no allowance is needed, a null address of `0x0000000000000000000000000000000000000000` is then returned instead.
 
+Refer to this guide for [how to set your token allowances](https://0x.org/docs/guides/how-to-set-your-token-allowances). 
+
 ### Addresses by Network
 
 The following table includes commonly used contract addresses. For a full list of our smart contract deployments address, see the [0x cheat sheet](https://0x.org/docs/guides/0x-cheat-sheet).