docs: add "Why?" section to readme

README.md

@@ -3,6 +3,23 @@
 S3PyPI is a CLI for creating a Python Package Repository in an S3 bucket.
 
 
+## Why?
+
+The official [Python Package Index (PyPI)](https://pypi.org) is a public
+repository of Python software. It's used by `pip` to download packages.
+
+If you work at a company, you may wish to publish your packages somewhere
+private instead, and still have them be accessible via `pip install`. This
+requires hosting your own repository.
+
+S3PyPI enables hosting a private repository at a low cost. It requires only an
+S3 bucket for storage, and some way so serve files over HTTPS (e.g. CloudFront).
+
+Publishing packages and index pages to S3 is done using the `s3pypi` CLI.
+Creating the S3 bucket and CloudFront distribution is done using a provided
+Terraform configuration, which you can tailor to your own needs.
+
+
 ## Alternatives
 
 - [AWS CodeArtifact](https://aws.amazon.com/codeartifact/) is a fully managed