Document build.js issue with deleted symbols

I reported this issue in https://groups.google.com/d/msg/ol3-dev/bMEyHJAY0Is/JzK5OqJDiQUJ and it has just come up again after #2159 which removed control.Logo. It only occurs when symbols are removed from the api, which won't happen very often. There is an easy work-around, so I'm not sure it makes sense working on a  software fix; on the other hand, the problem is likely to baffle those running build.js, so I'd suggest adding a comment to the readme.