Added Arch instructions to VPS.md

* Added list of needed Arch packages
* Added Arch specific instructions 
* Switched to using useradd instead of adduser as it is more distro agnostic (adduser is not
* Instructed users to always use '-l' switch when they 'su'
Fixed Typo
Merge pull request #291 from mickstephenson/patch-1

Added Arch instructions to VPS.md