user-guide: add info about --no-scm mode (and link from get-started)

[efiop]

Maybe worth making some portions of current get-started optional. Especially with the upcoming Hg support.

UPDATE: Skip to #100 (comment) for more details.

[shcheklein]

Should be part of the user-guide. A link in the get started should be enough. It's not a prevalent use case to distract attention on it.

[algomaster99]

@shcheklein Is this issue about dvc init --no-scm command? How does it change the workflow? For example, dvc diff wouldn't work since the project won't have any commits.

[shcheklein]

@algomaster99 yes, some commands won work. I would say, it a very low priority issue. Let's not focus on this one for now.

[dashohoxha]

Working on a Katacoda scenario for this: https://katacoda.com/dvc/scenarios/data-management

[dashohoxha]

I have created a lesson/tutorial about using DVC in --no-scm mode: https://katacoda.com/dvc/courses/basics/data
Is this sufficient to close the issue?

[shcheklein]

@dashohoxha not enough, I believe. I think this ticked should be implemented as a note/mention in one of the introductory user guide sections (that are still not implemented and probably will be taken from the Understanding DVC). I can imagine there can be a section named "How it works" that can have a page inside "DVC and Git" - just an example to give you some sense. We should come with better titles for this.

Bottom line, I believe it'll be closed as part of #425, and #144, and #550.