Add .clang-format

This tells clang-format what kind of style we want to format as, see
https://clang.llvm.org/docs/ClangFormatStyleOptions.html.

Set it based on Google style. We can iterate on details in the
future.

.clang-format

@@ -0,0 +1,7 @@
+BasedOnStyle: Google
+AlignAfterOpenBracket: AlwaysBreak
+DerivePointerAlignment: false
+PointerAlignment: Left
+QualifierAlignment: Left
+ReferenceAlignment: Left
+InsertBraces: true