Exclude jsbundle files from VCS

Summary:
jsbundle files can be generated, and are quite large and therefore, I think should be excluded from being committed to the repo.

[ GENERAL ][ MINOR ][local-cli/templates/_gitignore] - Included a new entry to ignore jsbundle files
Closes facebook/react-native#17888

Differential Revision: D6977064

Pulled By: hramos

fbshipit-source-id: 9c7803004f3f4ec59cba3017213f68fba8225dbf

Author: aneophyte

templates/HelloWorld/_gitignore

@@ -51,3 +51,6 @@ buck-out/
 */fastlane/report.xml
 */fastlane/Preview.html
 */fastlane/screenshots
+
+# Bundle artifact
+*.jsbundle