Add c-code section to python style guide

[stscicrawford]

@eteq:

We should probably mention something about wrapping C code. I don't think we want to be overly-perscriptive, but maybe something like "Try Cython because it's easier for others to read, supports numpy (use the memoryview technique for that) and builds with a python project mostly transparently. If that doesn't work, try other reasonably standard tools like pybind. If that doesn't work, use the C-API directly but use existing build machinery instead of rolling your own if at all possible."?