Announce caffe-users mailing list

The Caffe community could use a mailing list to discuss usage, installation, and questions while focusing development discussions on GitHub. The [caffe-users mailing list](https://groups.google.com/forum/#!forum/caffe-users) is the place to talk.

Everything about usage, installation, applications, and so on should be discussed on the mailing list from now on.

Soon issues opened for typical mailing list discussions will be closed and redirected to the mailing list.

Thanks @yosinski for the suggestion in #765 and thanks @jeffdonahue for creating the list.