Update README.md

Add more detailed description for Polygon.io.

README.md

@@ -768,7 +768,7 @@ API | Description | Auth | HTTPS | CORS |
 | [Nordigen](https://nordigen.com/en/account_information_documenation/integration/quickstart_guide/) | Connect to bank accounts using official bank APIs and get raw transaction data | `apiKey` | Yes | Unknown |
 | [OpenFIGI](https://www.openfigi.com/api) | Equity, index, futures, options symbology from Bloomberg LP | `apiKey` | Yes | Yes |
 | [Plaid](https://plaid.com/) | Connect with user's bank accounts and access transaction data | `apiKey` | Yes | Unknown |
-| [Polygon](https://polygon.io/) | Historical stock market data | `apiKey` | Yes | Unknown |
+| [Polygon.io](https://polygon.io/) | Real-Time and Historical stocks, options, index and crypto market data | `apiKey` | Yes | Unknown |
 | [Portfolio Optimizer](https://portfoliooptimizer.io/) | Portfolio analysis and optimization | No | Yes | Yes |
 | [Razorpay IFSC](https://razorpay.com/docs/) | Indian Financial Systems Code (Bank Branch Codes) | No | Yes | Unknown |
 | [Real Time Finance](https://github.com/Real-time-finance/finance-websocket-API/) | Websocket API to access realtime stock data | `apiKey` | No | Unknown |