Add TODO for subtraction

Change-Id: Id41c76eb7b651f83ec29aea373ffebf02cf4ea16

src/main.js

diff --git a/src/main.js b/src/main.js
index 7d65831..8d462ee 100644
--- a/src/main.js
+++ b/src/main.js
@@ -1,3 +1,5 @@
 export function add(a, b) {
   return a + b;
 }
+
+// TODO: subtraction
\ No newline at end of file