Add Google-style .clang-format file.
It would be nice if the repository contained a clang-format
config file so that PR submitters could use tooling to correctly
format their code match Googles/abseils style.
This also makes it easier for reviewers to specify what coding style
should be used. ie the one the tooling produces.
The clang-format file is the default Google configuration as dumped by `clang-format`.
I'm not sure abseil will want the exact same options, but it seemed like the right
starting place.
diff --git a/.clang-format b/.clang-format
new file mode 100644
index 0000000..06ea346
--- /dev/null
+++ b/.clang-format
@@ -0,0 +1,4 @@
+---
+Language: Cpp
+BasedOnStyle: Google
+...
