Update README for v2.0
I forgot to update the README for v2.0.0.
The important changes were in v2.0.0.
Switch from Rusoto to aws_sdk
One known breaking change: different choices for the --format option: csv (the default) or json.
The underlying AWS library switched from Rusoto to the not-yet-v1 aws_sdk. There could easily be different behaviors from the newer library. I did not see anything glaringly different.