add PM2.5 Open Data Portal in Environment section

PM2.5 Open Data Portal provides open data from the AirBox project, which is a low-cost PM2.5 sensing project. The data frequency is every 5 minute, and there have been more than 15,000 sensors deployed worldwide.

README.md

@@ -320,6 +320,7 @@ API | Description | Auth | HTTPS | CORS |
 | [GrünstromIndex](https://www.corrently.de/hintergrund/gruenstromindex/index.html) | Green Power Index for Germany (Grünstromindex/GSI) | No | No | Yes |
 | [OpenAQ](https://docs.openaq.org/) | Open air quality data | `apiKey` | Yes | Unknown |
 | [PM25.in](http://www.pm25.in/api_doc) | Air quality of China | `apiKey` | No | Unknown |
+| [PM2.5 Open Data Portal](https://pm25.lass-net.org/#apis) | Open low-cost PM2.5 sensor data | No | Yes | Unknown |
 | [PVWatts](https://developer.nrel.gov/docs/solar/pvwatts/v6/) | Energy production photovoltaic (PV) energy systems | `apiKey` | Yes | Unknown |
 | [UK Carbon Intensity](https://carbon-intensity.github.io/api-definitions/#carbon-intensity-api-v1-0-0) | The Official Carbon Intensity API for Great Britain developed by National Grid | No | Yes | Unknown |