Explain thick bordered node in N2G

[albertsgarde]

Some nodes in N2G are marked (currently with thick borders) as end nodes. The meaning of this should be explained efficiently to the user.

This issue should probably be solved as a part/extension to #105

Explanation of end nodes

These nodes are valid path beginnings. That is, a string of tokens starting with one of these nodes, and including all tokens on a path to a red node, in order, will activate the neuron.
E.g., you could have a neuron that activates on “Alex” in “My name is Alex” as well as in “name is Alex”. In that case both “My” and “name” are valid start nodes (and will be marked), but “is” isn’t.