Add 'Installing mods with DTMM'

Installing-mods-with-DTMM.md

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+# Installing mods
+
+1. Ensure that the "Data Directory" and "Game Directory" in the "Settings" tab are set properly.
+2. Download the mod's release archive (a `.zip` file).
+3. Click "Add mod" in DTMM and select the file you downloaded.
+4. Use the checkbox in the mod list to enable the new mod.
+5. Click "Deploy mods" and watch the log at the bottom until it completed.
+6. Start the game normally.
+
+## Handle game updates
+
+After the game was updated, mods need to be deployed again. Simply click "Deploy mods" and watch the log view at the bottom, as usual.
+
+## Migrating from dtkit-patch
+
+_NOTE: This includes installations managed manually or with Vortex._
+
+1. Copy the `mod_load_order.txt` file from the game files somewhere else, for reference later
+2. Click "Reset Game"
+3. Download the newest versions of your mods and install them, following the steps above
+
+# Troubleshooting
+
+If you've run into issues or received an error message during installation or game deployment, check the sections below.
+If those didn't help, you'll always find helpful people in the [Modders Discord](https://discord.gg/GFyCGNpJb8).
+
+When reporting issues, please attach a screenshot of the error message and DTMM's log file. You can find the log by entering `%APPDATA%\dtmm` into the File Explorer's address bar.
+
+## "Found dtkit-patch-based mod installation"
+
+Follow the migration steps above.
+
+## "Failed to open game bundle directory"
+
+DTMM needs to know where to find the game files. Double-check the "Game Directory" value in the "Settings" tab, and make sure it points to the game's installation directory, e.g. `E:\SteamLibrary\steamapps\common\Warhammer 40000 Darktide`.
+
+## "Failed to import mod"
+
+DTMM requires the mod archives to be built in a defined way. If importing fails for a mod, it might still only support the old ecosystem.
+Check if there is a newer version. If not, try to get in touch with the author and point them to [Migrating from loose files](Migrating-from-loose-files) and [Building your first mod](Building-your-first-mod). The [Modders Discord](https://discord.gg/GFyCGNpJb8) is a good way to get in touch with the authors, or other people that can help you out.
+
+## Mod dependency issues
+
+> "Dependency XXX must be enabled"<br>
+> "Dependency XXX must be loaded before YYY"<br>
+> "Dependency XXX must be loaded after YYY"<br>
+
+One of your mods specified another mod as dependency, optionally with a required order. Check the error message, and the mod's description and adjust the order accordingly, with the "Move Up" and "Move Down" buttons.
+