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 now reads like the user has to be in `~/.dotfiles` in order to use rcup(1), which is untrue! How about something along the lines of:

`In the other direction you can use rcup(1) to create the symlinks from ~/.dotfiles to your home directory.`