Added missing requirements

Issue #254 requests that the missing required functions `exec`, `shell_exec`, and `escapeshellarg` be added to the README.

This pull request does that by adding an extra step under 'Installation'.

This is also my first ever pull request on github.

Screenshot:
![linux-dash commit screenshot](https://cloud.githubusercontent.com/assets/9929737/5234104/b4762aa4-77ad-11e4-91b9-53662973d57b.png)