Add a tutorial on how to create a map

mods/game/maps/README.md

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+# How to create a map:
+
+Requirements:
+[Minetest/Luanti 5.0.0 or later](https://luanti.org/)
+[Minetest Game](https://content.luanti.org/packages/Luanti/minetest_game/)
+[WorldEdit](https://content.luanti.org/packages/sfan5/worldedit/)
+[Simple Shooter Game](https://github.com/a-bad-dev/simple-shooter-game/)
+
+Step 1: Build your map. It must be perfectly square or rectangular and not contain any non-MTG nodes or rails.
+Step 2: Surround the playable area in a box made of glass, WorldEdit helps with this.
+Step 3: Create a barrier 4 blocks below the top of the map out of obsidian glass, leaving a 3-block gap of air between the barrier and roof.
+Step 4: Select opposite corners of the map with WorldEdit and run `//mtschemcreate map`.
+Step 5: Create a folder with the titile of your map in all lowercase with no spaces in `simple-shooter-game/mods/game/maps/maps/`.
+Step 6: Copy the file `(Minetest/Luanti install path)/worlds/(name of the world in which you created your map)/schems/map.mts` to `simple-shooter-game/mods/game/maps/maps/(your map's folder)/`.
+Step 7: Create a file called `map.lua` in `simple-shooter-game/mods/game/maps/maps/(your map's folder)/`.
+Step 8: Open the `map.lua` file in a text editor and put the following content in it:
+```lua
+local map_data = {
+	name = "(Your map name here)",
+	size_x = (Size in the X direction of your map),
+	size_y = (Size in the Y direction of your map),
+	size_z = (Size in the Z direction of your map),
+	
+	barrier_level = (Distance from the bottom of the map to the barrier),
+	
+	spawn_x = nil,
+	spawn_y = nil,
+	spawn_z = nil
+}
+
+return map_data
+```
+Step 9: Open Minetest/Luanti and create a new world with Simple Shooter Game.
+Step 10: Play the world and run `/load (your map name)` followed by `/start`.
+Step 11: Verify the map loads and works.
+
+And that's it!