ADD new tutorials on ROC to tutorials list

[iagomosqueira]

Two new tutorials on length-based methods need to be added to the tutorials section of the main page.

• https://github.com/flr/doc/blob/master/length_based_methods.Rmd
• https://github.com/flr/doc/blob/master/length_based_methods_classification_skill.Rmd

Any errors, please report to @laurieKell