Add 'Migrating from loose files'

Migrating-from-loose-files.md

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+> **NOTE**: If this is your first time migrating a mod, I strongly suggest going through [Building your first mod](Building-your-first-mod) once, to play around with and get a feel for the new file structure.
+
+To migrate Lua files from a loose file structure to something that can be built into mod bundles with DTMT, the following is needed:
+
+- a `dtmt.cfg` that defines the mod's metadata, entry points and packages
+- at least one `.package` file that includes your Lua files
+
+Use the following template to create your `dtmt.cfg`:
+
+```sjson
+// The ID string you pass to `get_mod()`
+id = ""
+// The human-readable name of the mod. This will show up in places like DTMM's mod list and DMF's options view
+name = ""
+// A summary of what your mod does. Shows up in DTMM
+description = ""
+// A version number to track changes in your mod with. Not used, yet, but good practice to always have anyways.
+version = ""
+
+// The Lua files passed in the second parameter of DMF's `new_mod`
+resources = {
+	// The Lua file to be called by DMF when it initializes your mod.
+    init = ""
+    data = ""
+    localization = ""
+}
+
+packages = [
+	// At least one package file to load your files. It doesn't have to be named after your mod's ID value, but it is good practice to do so.
+    "packages/<id>"
+]
+```
+
+Please note that all file paths should be specified _without_ file extension. The `resources` section can be adapted from the contents of your `.mod` file. All instances of `<id>` are intended to be replaced with the value you specify in `id = ""`, i.e. the value passed to DMF's `new_mod` and `get_mod`.
+
+The `.package` file supports wildcards, so the most simple version would look like this:
+
+```sjson
+lua = [
+	"scripts/mods/<id>/*"
+]
+```
+
+A suggested final file structure looks as follows:
+
+```none
+<path/to/new/folder>
+└──<id>
+   ├──dtmt.cfg
+   ├──packages
+   │  └──<id>.package
+   └──scripts
+      └──mods
+         └──<id>
+            ├──data.lua
+            ├──init.lua
+            └──localization.lua
+```
+
+All additional Lua files can then be dropped into `scripts/mods/<id>` (sub-directories are supported) and can be `require`d in-game with the full path, e.g. `require("scripts/mods/<id>/my_extra_file")`.
+
+Modding veterans from VT2 may recognize this file structure 🙂