Add information about source linked PDB files

CHANGES.md

@@ -6,6 +6,10 @@ like parallelism, timeouts, directory and path based asserts, etc. It also conta
 numerous bug fixes and smaller enhancements. We've improved our XML docs,
 fixed performance issues and added more detail to Multiple Asserts.
 
+This release also contains source linked PDB files allowing developers to debug
+into the NUnit Framework. This allows you to track down errors or see how the
+framework works.
+
 In order to support the .NET Standard 2.0 version, the NUnit project switched to
 the new CSPROJ format and now requires Visual Studio 2017 to compile. This only
 effects people contributing to the project. NUnit still supports building and