add a note about reflinks and how space is calculated in the tutorial

[efiop]

In our article https://dvc.org/doc/tutorial/define-ml-pipeline , we use du to show that space taken by the repo didn't double. That works with hardlinks, but du is not smart enough when it comes to reflinks, which it counts as separate full-blown filse and accounts for their size two times(for the cache file and for the reflink), which makes it confusing for the mac users iterative/dvc#1442 . We should add a note about it.

[efiop]

Note: to prove that no duplication occurs, we could use df utility to see that free space on the drive indeed didn't decline by the file size that we are adding.

[kurianbenoy]

Can I work on this issue?

[shcheklein]

@kurianbenoy absolutely, feel free to start working on this! Join the dvc.org/chat and #dev-docs channel if you need any help.

[kurianbenoy]

Add docs in tutorials about various link types in this page as well when documentation is ready for this.

[ghost]

Someone got confused on Discord because the tutorial implies a cache type of hardlink / symlink / reflink
https://discordapp.com/channels/485586884165107732/485596304961962003