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>

.clang-format

@@ -1,4 +1,8 @@
-# Defaults
+# This file is an example configuration for clang-format 5.0.
+#
+# Note that this style definition should only be understood as a hint
+# for writing new code. The rules are still work-in-progress and does
+# not yet exactly match the style we have in the existing code.
 
 # Use tabs whenever we need to fill whitespace that spans at least from one tab
 # stop to the next one.