Add t2 spinal cord data (#13)

* Add t2 spinal cord data

* Add provenance of data

* Update README with specific release

* Update t2/README.md with specific file

Co-authored-by: Julien Cohen-Adad <jcohen@polymtl.ca>

Author: po09i

t2/README.md

@@ -0,0 +1,5 @@
+T2w data
+========
+
+This data originates from the Spinal Cord Toolbox Testing Data:
+https://github.com/sct-data/sct_testing_data/blob/r20210114/t2/t2.nii.gz