Add .clang-format file from CMSSW

This makes sure you're pickig up the right code style also when cloning
combine outside the CMSSW environment.

.clang-format

@@ -0,0 +1,17 @@
+---
+Language: Cpp
+BasedOnStyle: Google
+ColumnLimit: 120
+NamespaceIndentation: All
+SortIncludes: false
+IndentWidth: 2
+AccessModifierOffset: -2
+PenaltyBreakComment: 30
+PenaltyExcessCharacter: 100
+AlignAfterOpenBracket: Align
+AllowShortIfStatementsOnASingleLine: false
+AllowShortLoopsOnASingleLine: false
+AlwaysBreakTemplateDeclarations: Yes
+ReflowComments: false
+BinPackArguments: false
+BinPackParameters: false