Clarify node order in README

[ngoctnq]

sorobn/sorobn/bayes_net.py

Lines 320 to 321 in 5e14193

	# The nodes are sorted in topological order. Nodes of the same level are sorted in
	# lexicographic order.

Can we have a clear mention of this in the README? I was wondering about the difference in node ordering -- thought it was in order of model definition, but couldn't find this out until I dug into the code.