Add in README or WIKI link to spinalcordtoolbox/dev commit to the scripts/files used to generate the WM atlas

[jcohenadad]

It would not only be useful for reproducibility/traceability, but also as ressource for related project (eg: #46 (comment))

Here's the link (grabbed from recent SCT meeting):
https://github.com/spinalcordtoolbox/PAM50/blob/61dfb41ca01ffb24079e4c942553fe93323e3883/scripts/create_atlas

[joshuacwnewton]

PR opened. :)