If it was just me, I’d use plain text files like people are suggesting in this thread. In particular, if you want to be able to track and do retrospectives of what’s been done, consider using jrnl[0] to log what you’ve done.

For small teams, I really like Dropbox Paper. It’s better than text files here because it’s real-time collaborative, but retains nearly all of the flexibility. It supports Markdown syntax, checkboxes, etc. You can tag members of your team, comment in the margins, and link to one doc quickly from another. For small teams that don’t need a heavyweight process, Dropbox Paper fills fits my needs here rather well.

[0] http://jrnl.sh