[clang-tidy] Provide links to the google style guide for checks derived from it.

llvm-svn: 213233
diff --git a/clang-tools-extra/clang-tidy/google/UsingNamespaceDirectiveCheck.h b/clang-tools-extra/clang-tidy/google/UsingNamespaceDirectiveCheck.h
index 3410666..940abb6 100644
--- a/clang-tools-extra/clang-tidy/google/UsingNamespaceDirectiveCheck.h
+++ b/clang-tools-extra/clang-tidy/google/UsingNamespaceDirectiveCheck.h
@@ -18,6 +18,7 @@
 
 /// \brief Finds using namespace directives.
 ///
+/// http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml?showone=Namespaces#Namespaces
 /// Corresponding cpplint.py check name: 'build/namespaces'.
 class UsingNamespaceDirectiveCheck : public ClangTidyCheck {
 public: