This is a plugin that allows you to create custom achievements with notification in UI with simple API

More about plugin and API you can read in addons/gd_achievements folder and in wiki

Now code for Godot 3.x.x is placed to godot3 branch and you should switch to this if you need it

Check that "addons/gd-achievements/scripts/achievements_manager.gd" file activated as singleton (check How to install in your project?)

You must add to your exporter .json format for non-resources export:

Export => Click on Android Presets => Go to Resources tab and set .json as filter for resources to export.

Thanks to Dark8Ghost for the found fix