While the game is very simple, I really like most of what you did here! It's a solid game that works and doesn't appear to have any bugs - so great job on that!
I do have a couple of suggestions though:
There's a slight oversight in the main menu, I think. You have the "help" screen there for people to learn the controls, but one has to already know the controls to get to the help screen. Granted, most players will be able to figure out to use spacebar, but I'm of the school of thought that the end-user shouldn't have to think or experiment to use a UI. So, it'd probably be best if you have a message like "press the 'spacebar' to select a menu option" somewhere on the main menu
Next, I would suggest adding a "game over" screen. It'd just be nice if the transition from dying and being sent to the menu was less abrupt, especially since there's no dying animation (which is fine for a game like this).
I would also suggest making the overall speed of the game (the bullet speed, player speed, enemy speed, etc...) faster. I suppose this is a personal preference thing, but I feel like it's a little too slow.
If you feel like experimenting with LOVE further with this game, a couple of features that'd be cool to add would be a score system, a lives system (including extra lives if the score reaches certain numbers), and a high score table. Different enemies would be fun too
