Bot should remind PR authors to add a description

When a PR is submitted without a description, the bot should remind people to add a description that describes why the PR is important to the ManageIQ codebase.

Possibly even include some rules around not just describing the changes, but describing the purpose of the changes.

The issue names no files, tests, or entry points. Start by locating where miq_bot handles newly submitted pull requests and detects missing descriptions; done means a PR without a description receives a reminder focused on its purpose and importance to the ManageIQ codebase.

Written by the indexing model from the issue text.