Add a code style rule to make all state variables private (#1390)

(cherry picked from commit 076cb7decd2a5823af23648105e2aa72888fb32a)

CODE_STYLE.md

@@ -16,6 +16,8 @@ Any exception or additions specific to our project are documented below.
 
 * Try to avoid acronyms and abbreviations.
 
+* All state variables should be private.
+
 * Private state variables should have an underscore prefix.
 
     ```