All you need is to use xdt:Transform="Insert"
attribute within a tag that you want added during transformation. Read more about it here: http://msdn.microsoft.com/en-us/library/dd465326.aspx
You can take the below sample as a starting point (which is my Web.Release.config
file):
<?xml version="1.0"?>
<!-- For more information on using Web.config transformation visit http://go.microsoft.com/fwlink/?LinkId=125889 -->
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
<!-- Enable static content caching in release mode -->
<system.webServer xdt:Transform="Insert">
<staticContent>
<clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="5.00:00:00" cacheControlCustom="public" />
</staticContent>
</system.webServer>
</configuration>