Need to rethink quickstart in future

[GodloveD]

Right now we are not actually covering anything except for the installation in this document. I think that is fine to go ahead and post to the website for now, just so we have something up. But in the future I would like to see the following changes.

• Only provide a single method (preferably RPM) for installing Singularity. Place all of the rest of the info in the installation document under the user guide and just link that guide here.
• Give a brief overview of the security context of Singularity (same user inside and outside of the container and priv separation model using NO_NEW_PRIVS). Provide links to more in depth info.
• Give a brief overview of the singularity.conf file. (Explain what it is, that it must be owned by root etc.) and then give links to more in depth info.

Slightly different idea for quickstart:

• A single installation method (preferably RPM) described quickly with a link to the user docs installation page for more info.

• A note about localstatedir

• A quick description of the singularity.conf file with a link to the appropriate page of the admin docs for more detailed info.

• A quick description of Singularity architecture (no daemon, security context, default namespaces, why architecture works with batch schedulers) with links to appropriate sections (probably in admin docs introduction?) for more info.