Added clarifying note about which OneCall API is used

README.md

@@ -55,7 +55,7 @@
        - [*"weather"*](https://openweathermap.org/current), the current weather
          data set
        - [*"onecall"*](https://openweathermap.org/api/one-call-api), the "One
-         Call API" data set
+         Call v2.5 API" data set (note: does *not* use the v3.0 API)
        - [*forecast*](https://openweathermap.org/forecast5), the 5-day/3-hour
          forecast data set
        - [*air_pollution*](https://openweathermap.org/api/air-pollution), the air