Added README

README.md

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+mapgen_rivers
+=============
+
+Procedural map generator for Minetest 5.x. Still experimental and basic.
+
+Contains two distinct programs: Python scripts for pre-processing, and Lua scripts to generate the map on Minetest.
+
+# Installation
+This mod should be placed in the `/mods` directory like any other Minetest mod.
+
+The Python part relies on external libraries that you need to install:
+- `numpy`, a widely used library for numerical calculations
+- `noise`, doing Perlin/Simplex noises
+- optionally, `matplotlib` (for map preview)
+
+They are commonly found on `pip` or `conda` Python distributions.
+
+# Usage
+## Pre-processing
+Run the script `terrain_rivers.py` via command line. You can optionally append the map size (by default 400). Example for a 1000x1000 map:
+```
+./terrain_rivers.py 1000
+```
+For a default 400x400 map, it should take between 1 and 2 minutes. It will generate 5 files directly in the mod folder, containing the map data (1.4 MB for the default size).
+
+## Map generation
+Just create a Minetest world with `singlenode` mapgen, enable this mod and start the world. The data files are immediately copied in the world folder so you can re-generate them afterwards, it won't affect the old worlds.