Update 'Download & Installation'

Download-%26-Installation.md

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+At this time, *Heck* is only available as a [PolyMC] / [MultiMC] instance that is set up to auto-update using [packwiz].
+
+## Prerequisites
+
+### PolyMC / MultiMC
+
+If you don't have one already, get either [PolyMC] or [MultiMC]. These are open source third-party launchers that can make it easier to manage multiple Minecraft instances that can each run different versions of the game and have different sets of mods. They're also configurable enough to allow us to hook into [packwiz] which then makes it possible to download mods from the authors directly, such as from [Modrinth] and [CurseForge], rather than having to redistribute a massive archive with all mods for every single change. Updates are quick, light, and most importantly, easy to create and manage from our end.
+
+If you're unsure which to pick, I would suggest [PolyMC], as it's in more active development with features (such as easy mod installation) you might find useful if you end up using it for your every-day Minecraft needs.
+
+### Java 17+
+
+Unfortunately, neither launcher currently handles installing a Java runtime for you, which is required to play Minecraft. Version 1.18.2 of the game requires Java 17 or above to run. Older versions may require older Java runtimes. This is why it's useful to know where to get them.
+
+To check which versions of Java are installed, click "Settings" in your launcher, navigate to the "Java" tab and click the "Auto-detect..." button. This will open a popup with all Java versions on your system. If you can already see the right version in this list, you can select it here and this step.
+
+If you want to download Java, I recommend [Temurin], an OpenJDK build by the Eclipse Foundation. Once the download is complete, the installation process should be pretty straight-forward. I don't think there's anything in particular I need to point out here.
+
+## Modpack Installation
+
+Finally, to get the *Heck* modpack added to your launcher, press "Add Instance", select "Import from zip", copy the modpack URL [`https://meowface.org/copygirl/Heck.zip`](https://meowface.org/copygirl/Heck.zip) into the textbox and click "OK".
+
+When launching, you will be asked which optional mods you want to use. I suggest to look up the mods online if you're unsure if you want to use them, or ask the community if anything is unclear. If you want to change your mind later, you can always do so when you start up the instance again.
+
+On first launch, [packwiz] will download all mods, configs and other associated files. And whenever you start the game, it will check for updates and apply them as well. That way you're always on the latest version of the modpack without having to worry about updating it manually.
+
+
+[PolyMC]:  https://polymc.org/
+[MultiMC]: https://multimc.org/
+[packwiz]: https://github.com/packwiz/packwiz
+
+[Modrinth]:   https://modrinth.com/
+[CurseForge]: https://curseforge.com/
+
+[Temurin]: https://adoptium.net/temurin/releases/