Add a `README.md` to the `SynthSeg/dcan/paper/` directory (from code review)

[GregConan]

Add a README.md to the SynthSeg/dcan/paper/ directory specifically, in addition to the README.md file at the top/root level of the repository.

• The README.md file should explain the usage of the script(s) in that directory.
• If the scripts are organized so that the user can run different combinations of 5 stages, as suggested in another issue, then the README.md should explain each stage and the differences between them.

This change was suggested during the code review on 2022-02-09.