add .clang-ignore

Add .clang-ignore file to avoid formatting outside sources.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: Ic412732d4bbd401dc1504717fbb00441d5b80817
diff --git a/.clang-ignore b/.clang-ignore
new file mode 100644
index 0000000..9376935
--- /dev/null
+++ b/.clang-ignore
@@ -0,0 +1,13 @@
+./boost-src
+./Boost-prefix
+./CMakeFiles
+./boost-src
+./nlohmann-json-prefix
+./nlohmann-json-src
+./nlohmann
+./prefix
+./sdbusplus-project
+./sdbusplus-src
+./valijson-prefix
+./valijson-src
+./valijson