Specify Cordova Android v8+ is required

[sjregan]

Feature Request

Specify Cordova Android v8.0.0+ is required in the readme.

Feature Description

Make it clear that this plugin will not install into the Android platform unless Cordova Android v8.0.0+ is installed. This is useful as it would save a lot of debugging time trying to figure out why the plugin works on iOS but not on Android (in certain circumstances).