Update CODE_STYLE.md (#1842)

Add: Interface names should have a capital I prefix.

CODE_STYLE.md

@@ -61,3 +61,9 @@ Any exception or additions specific to our project are documented below.
 
   Some standards (e.g. ERC20) use present tense, and in those cases the
   standard specification prevails.
+  
+* Interface names should have a capital I prefix.
+
+    ```
+    interface IERC777 {
+    ```