Final mods for initial CONTRIBUTING.md

There are still a few sections that need to be written or expanded on.
> compiler code must be really hard to look at

No, the compiler itself doesn't have to written that way! I didn't phrase it correctly -- I'll fix it. Any symbols that the compilation system (including system headers and libraries) introduces into your program use these ugly reserved names so that they don't conflict with names you might expect to be able to use in your program. So where the really, really ugly stuff is found is in the standard template library code like `<vector>` or `<map>`.