Add Polygon.io readme

Author: Romkisel

java/connectors/polygon-io/README.md

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+## Polygon.io Data Connector
+
+**Important**: This connector writes data for exchanges and conditions using their numeric identifiers as received. 
+To interpret these numeric identifiers, please use the following resources from Polygon.io:
+
+- For Exchange IDs: consult the [Exchanges](https://polygon.io/docs/crypto/get_v3_reference_exchanges) endpoint documentation.
+- For Condition IDs: reference the [Conditions and Indicators](https://polygon.io/glossary/conditions-indicators) glossary page.