Final mods for initial CONTRIBUTING.md

There are still a few sections that need to be written or expanded on.
> I prefer int i = 0 for consistency with assignments everywhere else.

A perfectly respectable point of view! Personally I don't think consistency with constructs elsewhere matters much in practice. I am more concerned with readability of the code I'm looking at and here I don't think the extra space buys anything, especially since the initialization is such a cliché in C++ for loops. In my view the only part of that `for` statement that has any meaningful content is the condition `i < length`; if I had to guess the initialization and increment code I would have guessed right.