Comment 3 for bug 769948

There's an API. Plugin gets "inserted" into process of installing packages. It is described here: http://yum.baseurl.org/wiki/WritingYumPlugins
There is also another way ( http://mo.morsi.org/blog/node/220 ), but it would need to be called with some interval, gather information about all installed packages and compare them with previous state, which isn't performance-wise.
Though there may be another way (with better performance, easier, etc.). If there is, please point it out.