Added Readme and screenshot

and moved 30-biomes.ggb to /diagram

README.md

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+# 30 Biomes for Minetest
+
+This mod is still experimental. It provides 30 main biomes and 10 beach/sea biomes for Minetest Game. Compatible with all core mapgens *except v6*.
+
+Biomes are all registered for now, but they are not all different for now, because there is not enough plants. Obviously there will be more plants in the future.
+
+![Screenshot](https://raw.githubusercontent.com/Gael-de-Sailly/30-biomes/master/screenshot.png)
+
+## Diagram
+* Horizontal axis: **temperature**
+* Vertical axis: **humidity**
+
+The values are arbitrary units that are generally between 0 and 100 but might exceed these limits.
+![Voronoi diagram](https://raw.githubusercontent.com/Gael-de-Sailly/30-biomes/master/diagram/voronoi_diagram.png)
+
+It was taken from GeoGebra, with the file `30-biomes.ggb` (GeoGebra diagram).
+
+## Data structure
+The properties of each biome are stored in `biomes.csv`. You would find it awfully unreadable if you open it with a text editor. Actually you should open it with a spreadsheet software like Calc or Excel. To modify it, simply sodify the cells and save the file with the same format.