We've been writing 'feature documents' as defined by Cucumber, github.com/cucumber/cucumber/wiki/Feature-Introdu…. This has been working extremely well as we are only trying to express the requirement, not use Cucumber to run testing. We've found that the terse and declarative way of summarizing a set of features has been helpful for all participants in the process, including QA testers, senior management and those asking for new work.
We keep these documents - simple text files - in the repository where the code that is being written, React.js components . Once you get the hang of it, a feature doc is quickly written.