[feature-request] Validate other things in the readme

[jazzsequence]

If we're using this to validate WordPress plugin readme's generally, perhaps we should broaden the scope to validate other things.

Possible "other things" include:

• does Tested up to exist? if so, we can assume this is a WordPress plugin readme
  • if it's a plugin readme file that includes that field, does it also include all the other fields that could/should be in a readme.txt (since we're assuming this is a mirror)?
• does a CHANGELOG.md exist?
  • if not, does a changelog exist in the README.md?
• does a readme.txt exist?
  • if so, does it include all the expected fields that could/should be included in a readme.txt?
  • does it include a changelog, install instructions and frequently asked questions?

This broadens the scope away from validate-readme-spacing and dips into the territory of validate-wordpress-readme instead.