Add postman collection

Author: jason-fox

README.md

@@ -30,10 +30,8 @@ git checkout NGSI-v2
 ./services start
 ```
 
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-| [![NGSI v2](https://img.shields.io/badge/NGSI-v2-5dc0cf.svg)](https://fiware-ges.github.io/orion/api/v2/stable/) | :books: [Documentation](https://github.com/FIWARE/tutorials.Custom-IoT-Agent/tree/NGSI-v2) |
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+| [![NGSI v2](https://img.shields.io/badge/NGSI-v2-5dc0cf.svg)](https://fiware-ges.github.io/orion/api/v2/stable/) | :books: [Documentation](https://github.com/FIWARE/tutorials.Custom-IoT-Agent/tree/NGSI-v2) |  <img src="https://cdn.jsdelivr.net/npm/simple-icons@v3/icons/postman.svg" height="15" width="15"> [Postman Collection](https://fiware.github.io/tutorials.Custom-IoT-Agent/)
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