This is addon announces in chat when you interrupt a spell. It is lightweight and fully configurable.

To bring up the configuration interface, type /ai or /announceinterrupts. You can also reach it though the interface options.
In the configuration you can choose where you want the addon to be active, as well as the message to be printed when you interrupt and which channel to print it in.
By default the addon will not print messages outdoors and only inside instances areas (like raids, 5-mans, BGs, arenas, etc), this can be changed in the configuration options.