Add a brief README.md introduction

README.md

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+# QField Weather Forecast Plugin
+
+This [QField](https://github.com/opengisch/QField/) plugin delivers weather
+forecast directly on top of the map canvas, giving users an up-to-date
+overview of the weather to come while working in the field.
+
+The plugin utilises [Open-Meteo](https://open-meteo.com/)'s open source and
+free to use weather API.