Add firstrun.sh

This script is just for convenience.
I will run this script on every initial checkout of this repo.

For example: I would like to have my vimfiles at a remote server.
So I do this:

- `cd`
- `mkdir git-repos && cd git-repos`
- `git clone https://github.com/walialu/vimfiles.git && cd vimfiles && sh firstrun.sh`
- that's it we are done :)
Add setup script

This script will make my life a lot easier.
When I do a fresh checkout of my repo on a new machine I can now simply do:

- `cd && mkdir git-repos && cd git-repos`
- `git clone https://github.com/walialu/vimfiles.git` cd vimfiles && sh firstrun.sh`
Add install instructions