Explain what Illume is in the readme

[jamierajewski]

For those not immediately familiar, explain that it is an HPC cluster used for academic computing (particularly, particle physics). The architecture should be general enough that others who wish to use it can do so for other types of HPC, particularly on OpenStack (or with a bit of a rework, on other infrastructure like AWS that Terraform supports).

[jamierajewski]

Closed by 15c0214