guideline: don't break the workspace

[skshetry]

This is a Linux equivalent principle/guideline for DVC. I mentioned this briefly in yesterday's team meeting, posting it here for everyone's attention.

DVC is not really good in terms of following this, and can result in data loss for the users:

1. dvc add deletes stage files before even add starts.
2. Same issue with import/import-url/run/stage add.
3. DVC when committing the cache, moves the files to the cache and then after that is complete, relinks the file back to the workspace. This might result in data loss if it happens to fail in between.
4. DVC should be extra-cautious when checking out files that haven't been tracked? I am not sure about this though.

If you have areas where we are not following these, I can add it here and fix it as we go.