Added warning message for todo.sh archive

[chrysle]

I have added a warning message which todo.sh archive will print out if it does not find done tasks in the todo file. Before, it would even then print TODO: ./todo.txt archived.

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• [x] Fork the repository and create your branch from master.
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[chrysle]

Should I squash my commits?

[inkarkat]

You can squash if you want to, but there's no need. The change is looking fine now, thanks a lot!

It'll probably still take some time (at least until January) until this gets merged, we currently have a bit of a backlog of PRs, but @karbassi has a New Year's resolution of devoting some time to this project, and we'll very likely also do a fresh release then ;-)