Merge pull request #435 from jlin27/development

Update docs for skipValidation and Allowance Targets & Addresses

Author: jlin27

mdx/api/index.mdx

@@ -16,6 +16,7 @@ We offer hosted versions for different Ethereum networks. Requests for more netw
 | Avalanche           | https://avalanche.api.0x.org/ |
 | Fantom              | https://fantom.api.0x.org/    |
 | Celo                | https://celo.api.0x.org/      |
+| Optimism            | https://optimism.api.0x.org/  |
 
 ## Versioning
 
@@ -58,6 +59,32 @@ And will only document the objects in the `records` field.
 If a query provides an unreasonable (ie. too high) `perPage` value, the response can return a validation error as specified in the [errors section](#errors).
 If the query specifies a `page` that does not exist (ie. there are not enough `records`), the response will return an empty `records` array.
 
+
+## Allowance Targets
+Some interactions with 0x require or are improved by setting [token allowances](https://tokenallowance.io/), or in other words, giving 0x's smart contracts permission to move certain tokens on your behalf. Some examples include - 
+
+* Submitting a 0x API quote selling ERC20 tokens, you will need to give an allowance to the contract address. This address can be found either as the value of `allowanceTarget` returned in the quote response or in the ExchangeProxy Address column in the (#addresses by network) table below.
+* Trading ERC20 tokens using the Exchange contract, you will have to give an allowance to the [ERC20Proxy](https://0x.org/docs/guides/v3-specification#assetproxy) contract. 
+* 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). 
+
+| Network             | Endpoint                      | ExchangeProxy Address                      | ERC20Proxy Address                         | StakingProxy Address                       |
+|---------------------|-------------------------------|--------------------------------------------|--------------------------------------------|--------------------------------------------|
+| Mainnet             | https://api.0x.org/           | 0xdef1c0ded9bec7f1a1670819833240f027b25eff | 0x95e6f48254609a6ee006f7d493c8e5fb97094cef | 0xa26e80e7dea86279c6d778d702cc413e6cffa777 |
+| Binance Smart Chain | https://bsc.api.0x.org/       | 0xdef1c0ded9bec7f1a1670819833240f027b25eff | 0x0000000000000000000000000000000000000000 | 0x0000000000000000000000000000000000000000 |
+| Ropsten             | https://ropsten.api.0x.org/   | 0xdef1c0ded9bec7f1a1670819833240f027b25eff | 0xf1ec7d0ba42f15fb5c9e3adbe86431973e44764c | 0x6acab4c9c4e3a0c78435fdb5ad1719c95460a668 |
+| Polygon             | https://polygon.api.0x.org/   | 0xdef1c0ded9bec7f1a1670819833240f027b25eff | 0x0000000000000000000000000000000000000000 | 0x0000000000000000000000000000000000000000 |
+| Avalanche           | https://avalanche.api.0x.org/ | 0xdef1c0ded9bec7f1a1670819833240f027b25eff | 0x0000000000000000000000000000000000000000 | 0x0000000000000000000000000000000000000000 |
+| Fantom              | https://fantom.api.0x.org/    | 0xdef189deaef76e379df891899eb5a00a94cbc250 | 0x0000000000000000000000000000000000000000 | 0x0000000000000000000000000000000000000000 |
+| Celo                | https://celo.api.0x.org/      | 0xdef1c0ded9bec7f1a1670819833240f027b25eff | 0x0000000000000000000000000000000000000000 | 0x0000000000000000000000000000000000000000 |
+| Optimism            | https://optimism.api.0x.org/  | 0xdef1abe32c034e558cdd535791643c58a13acc10 | 0x0000000000000000000000000000000000000000 | 0x0000000000000000000000000000000000000000 |
+
 ## Errors
 
 Unless the spec defines otherwise, errors to bad requests should respond with HTTP 4xx or status codes.
@@ -709,7 +736,10 @@ Nearly identical to [`/swap/v1/quote`](#get-swapv1quote), but with a few key dif
 
 ### Request
 
-Identical to the request schema for `/swap/v1/quote`, with one exception: the `skipValidation` parameter will always be considered to be `true`, even when a `takerAddress` has been specified.
+Identical to the request schema for `/swap/v1/quote`, with one exception: the `skipValidation` parameter - 
+* On Mainnet, `skipValidation` is always considered `true`, even when a `takerAddress` has been specified. Note that this will change soon to behave as it does on Ropsten. 
+* On Ropsten, `skipValidation` behaves identically to that on `/swap/v1/quote`. For details, see the `skipValidation` parameter in the [/swap/v1/quote Request](https://0x.org/docs/api#request-1) section.
+
 
 ### Response