vkr: format with clang-format

clang-format insists on using .clang-format. This commit adds docs/vkr-clang-format as a doc. To use it,

$ cd src
$ ln -sf ../docs/vkr-clang-format .clang-format
$ clang-format -style file -i vkr_*

I am open for suggestions for the exact config we want to settle on.