Improve the example in README.md

[yuxiaoy1]

There are three updates:

• drop the table before create it, to fix the UNIQUE constraint failed error when re-run the example code.
• change username example to frank to fit better with person's name.
• use shorthand scalars() method and print all usernames.

[davidism]

I'm fine with the scalars change. I think that needs to be applied throught the docs though, I wasn't familiar with all the new patterns yet when the docs were cleaned up. Everything else, while it makes sense, I don't think needs to be in the example.

[yuxiaoy1]

Updated as commented, please review.