Add a style guide

[ssteinbach]

Should cover:

• Python & C++ style
  • Python: PEP8
  • C++: a description of what goes into the clang-format file
• How the linters and formatters work for those two languages (pointers to the clang-format file for example)
• Be included in the readthedocs documentation

[meshula]

This commit, on the style-guide branch, https://github.com/PixarAnimationStudios/OpenTimelineIO/commit/418faedb744f9dcc0b95c30a4dc80382e71f8b01 contains a placeholder style guide where we can iterate.

[meshula]

committed draft