This is a very simple script, so don't expect something great.
You can change it, distribute, do whatever you like, I don't mind. But if you improve it, you could share it back ;)
To use, you have to export variables to use aws-cli, like
AWS_DEFAULT_REGION, etc. Then just call the script using
watch (or however you like) and giving the RDS instance name:
watch ./rds-top.sh <your instance name>