Dragonfly is currently in its early developmental phases. While you certainly can use it, there will be things that are lacking or unimplemented. If you encounter any bugs or would like to see a feature implemented, feel free to open an issue.
Due to the way Dragonfly functions, it doesn't support plugins. Dragonfly is a library that has to be included by another program, where the actual logic would be controlled, altered and extended by the main program.
If you're using Dragonfly as a library, you can simply do:
go get -u github.com/df-mc/dragonfly@master
This will download the latest stable Dragonfly tag. If you're targeting a specific tag, try:
go get -u github.com/df-mc/dragonfly@tag
You can also target a specific commit by using the same command by replacing the tag placeholder with the commit hash.