Add a STYLE_GUIDE.md

[hyena]

It should go in the project top level. May just point to another existing style guide with caveats/deltas e.g. https://google.github.io/styleguide/cppguide.html

If covering C++ style in terms of coding style is premature, a guide covering just code formatting is a fine start for this. Getting something in place where we can document decisions later as we make them is more important than answering every side case now. See https://github.com/Weasyl/weasyl/blob/master/STYLE_GUIDE.md for an example of a simple one I wrote for Weasyl.

Once done, add a new issue about finding a linter (e.g. http://clang.llvm.org/extra/clang-tidy/ looks promising but it may be a poor match for the project's infrastructure)