clarify naming scheme in CONTRIBUTING.md

[maelle]

via #417

[szhorvat]

Can you add a little bit more detail in the description of new issues so those of us who haven't read (or don't remember) every discussion can still follow what's going on?

[maelle]

It might be useful for new contributors to understand how function names were chosen. From the aforementioned issue,

" For graph constructors, for example, I have come across three main types, sample_, make_ and graph_from_* (see a reprex below). However, I have not been able to locate documentation outlining these categories (or categories for other types of functions)."

There are a few answers in #417 (comment)

In a comment later there's "The idea is that you have make_() for creating graphs in a deterministic way, and sample_() for sampling graphs from some ensemble. Both of these functions take a specification of what to construct and a list of optional modifiers"