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!

[chriskyndrid]

This would be great. I realize SQLx isn't an ORM, but there seems to be a ton of issues relating to inquires on it having ORM functionality. I see "ormx" mentioned. Perhaps mentioning a "possible" ORM solutions section that references a few packages that interop with SQLx could be beneficial to addressing questions for newcomers. Personally, I have never been evangelical about the use of ORM's and have spent many hours debugging ORM problems with generated SQL over the years, but, I do find them useful for simple CRUD operations, mapping, etc.

I think the most robust ORM I've observed that works with sqlx (uses sqlx-core) is rbatis.

Thanks for all the work on the excellent package that SQLx is!

[jplatte]

Working on a PR...