Add READMEs to all fixture apps

[ForestEckhardt]

As part of the effort to remove .Net 2.1 from our fixture @sophiewigmore created READMEs for how to generate all of the apps that needed to be updated. I think that this was a brilliant idea and I would love to see documentation on how to generate the apps in all of our fixtures!

[fg-j]

This is nearly done. We just need to add a readme for https://github.com/paketo-buildpacks/dotnet-publish/tree/main/integration/testdata/source_steeltoe_3.1

[ForestEckhardt]

I think we just need a link to the repo where we sourced this if we did