Add a help page for trees

[jamesaoverton]

Or trees need a help page. On the left we should have the tree with a minor change: prefer that the top-level said "Class" instead of "owl:Thing" (related to #25). On the right we should have a friendly introductory paragraph:

Hello! This is an ontology browser. An ontology is terminology system designed for both humans and machines to read. Click the links on the left to browse the hierarchy of terms. Terms have parent terms, child terms, annotations, and logical axioms. The page for each term is also machine-readable using RDFa.