Add Mempool (#2806)

README.md

@@ -381,6 +381,7 @@ API | Description | Auth | HTTPS | CORS |
 | [INFURA Ethereum](https://infura.io/product/ethereum) | Interaction with the Ethereum mainnet and several testnets | `apiKey` | Yes | Yes |
 | [Kraken](https://docs.kraken.com/rest/) | Cryptocurrencies Exchange | `apiKey` | Yes | Unknown |
 | [Localbitcoins](https://localbitcoins.com/api-docs/) | P2P platform to buy and sell Bitcoins | No | Yes | Unknown |
+| [Mempool](https://mempool.space/api) | Bitcoin API Service focusing on the transaction fee | No | Yes | No |
 | [MercadoBitcoin](https://www.mercadobitcoin.net/api-doc/) | Brazilian Cryptocurrency Information | No | Yes | Unknown |
 | [Messari](https://messari.io/api) | Provides API endpoints for thousands of crypto assets | No | Yes | Unknown |
 | [Nexchange](https://nexchange2.docs.apiary.io/) | Automated cryptocurrency exchange service | No | No | Yes |