clang-format: add a comment about the meaning/status of the

clang-format: add a comment about the meaning/status of the

Having a .clang-format file in a project can be understood in a way that
code has to be in the style defined by the .clang-format file, i.e., you
just have to run clang-format over all code and you are set.

This unfortunately is not yet the case in the Git project, as the
format file is still work in progress.  Explain it with a comment in
the beginning of the file.

Additionally, the working clang-format version is mentioned because the
config directives change from time to time (in a compatibility-breaking way).

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>