baby-shower: add subject

subjects/baby-shower/README.md

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+### Baby shower
+
+It's time to celebrate the arrival of your creation in this world ; let's organize a baby shower!
+
+But you already worked a lot to give birth, so now you need some help from others to set the party.
+You're going to use a JS library - meaning some code made by other people, made available for anyone to use. Very cool & helpful, isn't it?
+There is a huge amount of packages of any type - really any - and it's usually quite simple to install and use.
+The one we're going to use today is [canvas confetti][0].
+
+From your script, [`import`][1] the package as indicated in the documentation, and launch the function to pop confetti when the page loads.
+Then, [add an event listener][2] on [`keydown` event][3] that triggers the same confetti popping function any time a key of the keyboard is pressed.
+
+### Expected output
+
+This is what you should see in the browser:
+
+<!-- video screenshot -->
+
+### Notions
+
+- [`import`][1]
+- [`addEventListener`][2] / [`keydown` event][3]
+
+[0]: https://www.skypack.dev/view/canvas-confetti
+[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import
+[2]: https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener
+[3]: https://developer.mozilla.org/en-US/docs/Web/API/Element/keydown_event#javascript