Add example for arbitrary columns

[Rmarken5]

During the development of my project, I came across the need to execute a multi-table join using sqlx.This resulted in arbitrary column names in the result set. I aimed to scan the resulting rows into a slice of structs containing embedded structs with db tagging.

Unfortunately, I was unable to find any documentation for such a scenario. (I did see the "known issues" section of the readme)
After forking sqlx , replacing the package in my go mod file with my local version, debugging, and experimenting, I was able to achieve the desired result after a considerable amount of time.

In the interest of helping others who may encounter similar issues, I suggest extending the readme usage with an example directory that provides additional examples of not-so-straightforward scenarios. This will be beneficial for those adopting this package into their projects.

[dlsniper]

Hello, @ardan-bkennedy, and I recently stepped in to help maintain this project.
We are sorting the opened issues and pull requests. We'll look at the example provided and merge it if everything works as expected..
Thank you for your contribution.