App.config

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5.2" />
    </startup>
  <appSettings>
    <add key="hoge" value=""/>
  </appSettings>
</configuration>

App.Debug.config

<?xml version="1.0"?>
<!-- For more information on using app.config transformation visit http://go.microsoft.com/fwlink/?LinkId=125889 -->
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <appSettings>
    <add key="hoge" value="Debug" xdt:Transform="Replace" xdt:Locator="Match(key)"/>
  </appSettings>    
</configuration>

App.Release.config

<?xml version="1.0"?>
<!-- For more information on using app.config transformation visit http://go.microsoft.com/fwlink/?LinkId=125889 -->
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <appSettings>
    <add key="hoge" value="Release" xdt:Transform="Replace" xdt:Locator="Match(key)"/>
  </appSettings>
</configuration>