You’ll notice that there’s a whole section that is commented out with this comment:
To modify your build process, add your task inside one of the targets below then uncomment that target and the DisableFastUpToDateCheck PropertyGroup.
Other similar extension points exist, see Microsoft.Common.targets.
So you just need to follow the instructions and add a command inside one or both of the pre/post build event xml nodes. To add a simple copy a file named lib.dll from the output directory to your project directory after building you’d do something like:
<Exec Command="copy "$(OutDir)/lib.dll" "$(ProjectDir)/"" />
Note: the " is intentional so that the string gets quoted when it’s used by the copy command.