Give pointers to relevant repositories holding multiple datasets

[tvercaut]

• https://endovis.grand-challenge.org/
• https://opencas.webarchiv.kit.edu/
• http://camma.u-strasbg.fr/datasets
• http://hamlyn.doc.ic.ac.uk/vision/
• https://sites.google.com/site/sznitr/code-and-datasets

This can be an unstructured list at the end for example.

[luiscarlosgph]

Added as suggested.