Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -44,3 +44,15 @@ Stands for _**S**ervice**W**orker_.
 
 #### Denyaize
 Nyaizeを解除すること
+
+## Code style
+### Don't use `export default`
+Bad:
+``` ts
+export default function(foo: string): string {
+```
+
+Good:
+``` ts
+export function something(foo: string): string {
+```