Add rcup example of adding new rc files

It wasn't obvious that you could also use rcup with undotted files to add them
as new rc files.

There was also a typo.

What it looks like now:
![2 tmux 2014-11-24 17-11-22](https://cloud.githubusercontent.com/assets/602470/5174192/06e7c02c-73fd-11e4-85b4-50d9c6726926.jpg)

This is really the opposite of the above: mkrc(1) takes a rc file and moves it to the dotfiles dir, symlinking it back; rcup(1) symlinks a file from the dotfiles dir to a rc file. I think of them as going in the other direction of each other.

So I'd say "In the other direction" instead of "Alternatively". Does that make sense?