Washing and medieval Europe aren't exactly the best friends... And as wool garments generally aren't worn close to the body, there is only a need to clean their stains. Which means that you just put it in water and if you don't get the stain away, you can recolour it. I'm not sure what, if any, detergent they used then. In ancient Rome they collected pee to make some sort of ammoniac detergent, but I doubt that was something used on the countryside and I'm insecure about if it's usable with wool. 

The problem with wool is that it breaks of too much heat or general harsh treatment. It also has to be dried carefully away from heat.

I think this say it better than I can:
http://www.care2.com/greenliving/wet-clean-wool-silk-and-rayon.html
http://www.kswpa.com/careofwool.htm