Sqlx: Add a brief explanation on how SQLx works under the hood to the readme

[jplatte]

This would be helpful to set expectations about what SQLx can and can't do, in addition to simply being interesting :)

Having a separate markdown file in the repo root instead of a README.md section could also work well, in particular if the brief explanation ends up not being so brief.

[abonander]

We should also have a big headline:

SQLx is not an ORM!