Add a Doxyfile for update_engine codebase

With a proper Doxyfile file, developer can execute `doxygen` and
automatically generate HTML files with documentation of functions,
graphs for inheritance hierarchy, function call graphs, etc.

Test: doxygen
Change-Id: I5679d8f6b6e3e7cc20b0a76ff4abed20f57704ab
diff --git a/Doxyfile b/Doxyfile
new file mode 100644
index 0000000..db31f86
--- /dev/null
+++ b/Doxyfile
@@ -0,0 +1,9 @@
+CLANG_DATABASE_PATH=../../
+HAVE_DOT=YES
+CALL_GRAPH=YES
+CALLER_GRAPH=YES
+GENERATE_HTML=YES
+GENERATE_LATEX=NO
+INPUT=.
+RECURSIVE=YES
+