There's a thread somewhere here about this vary topic, where such info can be posted, but I can't seem to find it. Try doing a search on "suppliers" or "services". But then again, is it alright with your mechanic to post his info on a public forum?
There aren't any express rules. I would just make sure that the mechanic is OK with his contact info being posted in a public forum. I usually say that I know of a shop that I would highly recommend, and ask that any interested parties communicate with me through private messages. There are two reasons for this. First, I don't want to be responsible for the shop being inundated by calls. Second, there is always the possibility that someone on this Board may not agree with you, and this could turn into a long discussion affecting the reputation of the shop, which doesn't even post on this BB. I'm not trying to tell you how to proceed, I am just suggesting some caution. By the same token, I would not hesitate to chime in on a thread where this would be applicable, and state how pleased you are with the work done for you.