Add .clang-format approximation

Add .clang-format approximation

Add .clang-format definitions that are an approximation of the current
coding style. As I understand it, the current style is based on what GNU
Emacs implements for C++. Hence these rules might not be entirely
accurate, but a good-enough approximation to allow contributers to
follow the coding style more easily.

I expect modifications for specific cases and when clang-format itself
evolves over time.

As of now, this definition is most useful in partial code formatting,
such as executed by `git clang-format` on staged files or
clang-format.py as a means of integration into various editors.

	* .clang-format: New File.

Signed-off-by: Matthias Maennich <maennich@google.com>