Add CODEOWNERS to prevent submodule issues

b8ef676615e230a09023e02ce4f0b0f8b1f6c2a1 accidentially touched a submodule. This would greatly diminish the chance of that happening and it landing on master again.
diff --git a/CODEOWNERS b/CODEOWNERS
new file mode 100644
index 0000000..e35f923
--- /dev/null
+++ b/CODEOWNERS
@@ -0,0 +1,2 @@
+# Prevent accidentially touching CSS submodules
+/css/tools/ @plinss @kojiishi @jgraham @gsnedders