Add a comment about the packnplay tests

Author: danieljbruce

handwritten/bigtable/system-test/install.ts

@@ -42,7 +42,7 @@ describe('📦 pack-n-play test', () => {
       sample: {
         description: 'JavaScript user can use the library',
         // Inject the reference directive at the top of the code
-        ts:
+        ts: // The reference header is needed so packnplay tests recognize require
           '/// <reference types="node" />\n' +
           readFileSync('./system-test/fixtures/sample/src/index.js').toString(),
         // Still needed so the package actually gets installed in the temp environment