How to add custom build step to a TFS Server Build ?

Most of the time when you are creating a build script (TFSBuild.proj), you need to do some steps after the build. Wether it's creating an MSI for easier deployment, creating a VSI for a Visual Studio Add-in, or whatever if may be... you normally do a post build. A post build event looks like the following inside the TFSBuild.proj