Add 'Building your first mod'

Building-your-first-mod.md

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+In a Command Prompt/terminal, run `dtmt new <path/to/new/folder>` to create a skeleton structure from the default template.
+During this, DTMT will prompt for several fields, including:
+   
+   - `name`: The human-readable name of your mod. This will show up in various places like DTMM's mod list or DMF's mod options view.
+   - `id`: A unique ID for your mod. The default is generated from `name` and should be good enough in most cases (clicking Enter without typing accepts the default value). This will be used throughout your code (e.g. in `get_mod()` calls).
+
+DTMT will now create the given directory from the mod template:
+
+```
+<path/to/new/folder>
+└──<id>
+   ├──dtmt.cfg
+   ├──packages
+   │  └──<id>.package
+   └──scripts
+      └──mods
+         └──<id>
+            ├──data.lua
+            ├──init.lua
+            └──localization.lua
+```
+
+You're now ready to start editing the `init.lua` file. For starters, you might want to just add a simple `print("Hello, world!")`, which will print this string in your console logs once you've loaded the mod.
+
+With the file edited, you now need to turn your mod into something that DTMM can load into the game. To do that, navigate to your mod's folder (where the `dtmt.cfg` is) and run `dtmt build`. Watch the output for any errors, but if all goes well, this will produce a `<id>.zip` file, which you can then import in DTMM and deploy.