Fix Google Drive permissions and update README

Fixes Google Drive permissions so only access to files that were created/opened* by the app can be accessed, in addition to readonly access to metadata.

Also updates the README to reflect new permissions, and fixes some spelling mistakes in the README.

*In this context "opened" has a special meaning, which is to open in the Google Drive website, or using their FilePicker GUI.  The important point is a file cannot be opened without explicit user knowledge/consent, so our app can't access any random file in Drive without the user knowing.  This should be explained in the README at some point.