"vi reqs.txt" is ideal default baseline

then only bump up to more complexity or superficiality as the benefits exceeds the cost/pain. for example, a spreasheet. perhaps a Google doc

if you're lucky enough to have any reqs/specs which have a natural machine-friendly form, like an assertion that X shall be <= 100ms then go ahead and express that in a structured way then write test code which confirms it, as part of a suite of assertions of all reqs which can be test automated like this