Improve Readme documentation

[adieuadieu]

Improve the readme.md documentation before 1.0 release.

• [ ] improve configuration section (specifically concerning outputs)
• [ ] improve documentation on usage of lib/sharp tarball
• [ ] add sample/example configuration files in ~/examples including how to setup with a single bucket, single existing bucket, etc.
• [ ] add a bit about what IAM permissions are required to deploy with Serverless (maybe link to the Serverless credentials documentation, too.)