No docs on how to use derive attribute in pep config

[khoroshevskyi]

After reviewing the PEP documentation with @mariamurach, we couldn't find information about derived attributes or how to add them to the PEP config. Consequently, this is confusing for users.

[nsheff]

Does this not satisfy?

https://pep.databio.org/spec/specification/#sample-modifier-derive

[mariamurach]

I think it covers a very complicated example very well, but I couldn't figure out from the documentation a simple use case of adding a path to file with a dynamic name based on other variables from sample table that's the same for all samples.