Update INSTALL.Unix

I am novice for db. I stuck with the 'sudo -i -u couchdb couchdb' line in First Run section. Becase I did not create couchdb user already. The shell keep asking to me couchdb's password. But I didn't know what is a problem. I was very annoying. Eventually I saw the next part and It describes user registration. After following it, I can run couchdb as couchdb user. So I suggest put User Registration (I changed section title..) section before First Run section.