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 user already. But I didn't notice what is a problem and I was very annoying. Eventually I saw the next part and It describes user registration. After doing that I can run couchdb as couchdb user. So I suggest put User Registration (I changed section title..) section before First Run section.