phew! neg rep is on the line,huh..
well anyway,here you go...
i don't think that madara told him about it,he only revealed the entire secrets of the uchiha tablet,which is the clans secrets,thus hashirama didn't told to anyone about it as he respected the promise and other clan's secrets.
now,for how he knows.Well it's because maybe madara wasn't the only one who took hashi's dna,the opposite also happened.Rinnegan is not unique to uchiha's.Maybe other 3 clans too can achieve it,by same method.Point to be noted,the person should be at the peak of it's respective clan's true limit then just exchange the dna with any of it's brethren clan,meaning either of uchiha and hyuga with either of uzumaki or senju.so maybe hashirama too had awaken rinnegan,just minute before death.we all know that as madara awaken,knowledge about it's techs. flew through his mind.the same must have happened to...