Add Readme

Author: passsy

arcgis_map_method_channel/README.md

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+# arcgis_map method channels
+
+[![Flutter Community: arcgis_map_method_channel](https://fluttercommunity.dev/_github/header/arcgis_map_method_channel)](https://github.com/fluttercommunity/community)
+
+[![pub package](https://img.shields.io/pub/v/arcgis_map_method_channel.svg)](https://pub.dev/packages/arcgis_map_method_channel)
+
+Platform channels for [`arcgis_map_ios`](https://pub.dev/packages/arcgis_map_ios) and [`arcgis_map_android`](https://pub.dev/packages/arcgis_map_android).