Document supported platforms and external dependencies

[csams]

We should update the README with the platforms insights-core can or should be able to run against as well as any external programs we need to have installed to run against the local machine or against archives. Here are a few of each off the top of my head. There may be more.

Platforms

This has more to do with how we curate specs and parsers.

• RHEL 6, 7, 8
• CentOS?
• Fedora?

External Programs

These are required for basic functionality.

• timeout
• GNU tar (opposed to BSD tar on mac)
• unzip
• file / libmagic