Document minimum system requirements

[kowen-rh]

Related to #112.

What are the minimum system requirements that must be met in order to run OpenShift in the VM? Other applications deployed may require more resources, but we should at least document what's required to get the baseline VM running.

[kowen-rh]

Minimum requirements for OpenShift 4 appear to be the following:

• 4 vCPUs
• 8GB RAM
• 20 GB storage space

The CPU and memory requirements are currently set as the defaults in pkg/crc/constants/constants.go.