Add command that figures out the path to this directory.

i.e., this command outputs the path from where the script ./install.sh
was run.

So lets say, if this repository was cloned in /home/username/dotfiles/,
then DIR will be /home/username/dotfiles/.

That means we don't need to hardcore it to ~/src/dotfiles/

This command was taken from this script:
https://github.com/mikeyk/dotfiles/blob/master/setup_symlinks.sh

Signed-off-by: Thiago Farina <tfarina@chromium.org>