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 https://github.com/facebook/react-native/pull/17888
Differential Revision: D6977064
Pulled By: hramos
fbshipit-source-id: 9c7803004f3f4ec59cba3017213f68fba8225dbf