Relevant literature to cite in README

[mattrmd]

It may be helpful to other users if the following relevant literature is cited in the README:

• Functional Models for Regression Tree Leaves | Torgo, 1997
• Functional Trees | Rodríguez et al., 2004
• An Experimental Study on Ensembles of Functional Trees | Gama, 2010
• Local Linear Forests | Friedberg et al., 2018