Update README explaining "migration issue" and what to do if repository appears to have issue

[andyfeller]

Overview

The ultimate output from the gh-repo-stat is whether there is a migration issue, which currently is if any of the following conditions occur:

1. Repository has more than 60,000 records
2. Repository is larger than 1.5 GB

Unfortunately, adopters of gh-repo-stats have no explanation of what to do if they encounter repositories that face this issue such as:

1. Deleting unwanted branches
2. Removing unnecessary content and compact the history

Suggestion

• Expand README.md documentation to include a section explaining the output from the extension including actions to take if an issue is found