AutoCombatLogging is a very simple addon that enables combat logging when you enter any raid or 5 man instance in the World Of Warcraft.
The file written is called WoWCombatLog.txt and can be found in your World of Warcraft\Logs\ folder most likely on your C drive.
AutoCombatLogging requires no configuration but every week or so I strongly advise to rename or delete this file as it can get very large if you do a lot of instances and raids. Unfortunately this process cannot be automated as addons are not allowed to interact with the file system.
NB: It seems that Lunarfall and Frostwall are classified as a 5 man instance and therefore AutoCombatLogging will activate when you enter your garrison. This is NOT a bug in the addon.