[docs] document the project's philosophy

[jameslamb]

It might be useful to document some of the high-level design choices in this project, including explanations of why those choices were made.

For example:

• any warnings being found result in a non-0 exit code
• all checks are evaluated before exiting
• all checks are accompanied by documentation explaining how to truly resolve them (not just how to get pydistcheck to stop reporting them)
• pydistcheck is not a library...so nothing should be imported from it
• dependencies are kept to an absolute minimum
• no pydistcheck-specific configuration files