XML Add Nodes

Have an XML file from a vendor and wanted to add a child node and having difficulty.

Since I cannot attach a file, I will try to detail the contents of the file.

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
.

.

.

</configuration>

I want to add this line right before the bottom </configuration>...

<runtime><generatePublisherEvidence enabled="false"/></runtime>

Any suggestions?