Documentation update with better examples of objects reading and editing

[isHarryh]

Summary

This PR updates the README file:

• Adds tips for properly closing files (use with and open() to manage file handle). We already have some issues like Can't move files after opening them #349 related to the new file-locking feature introduced in 1.23.0. The best way to avoid these issues is to improve our documentation.
• Adds details for object modification. Btw, fixes a outdated code that obj.save(instance) should be instance.save().

closes #349
closes #332