Update Readme.md to ease --diff usage comprehension and add relationships between Snowflake / permifrost grants

Merge Request Checklist

• Link to an issue
• Include the proposed fix or feature
• Include and update tests for the modified code
• Include a documentation change
• Add an entry in the Unreleased section of the CHANGELOG

(Only Readme.md so lot of checks unchecked 🤔 )

Update the Readme.md accordingly to our discussion @blake-enyart :

• add the table specifying what permifrost read/write grants are doing as snowflake grants
• add a use verbose comment to the --diff flag explanation
• fix a small typo

• closes #116 (closed)