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)

Good point! I'll make that change.