Add best practice docs on using TreeGrafter for obtaining GO annotations for non-reference species

[cmungall]

https://academic.oup.com/bioinformatics/article/35/3/518/5056037

For species that are not covered in panther, we recommend using treegrafter. Code and docker image here: https://github.com/pantherdb/TreeGrafter/

(should we also include this in future GO APIs?)