Add TerraBrush plugin as Heightmap editor (#333)

Co-authored-by: A Thousand Ships (she/her) <96648715+AThousandShips@users.noreply.github.com>

README.md

@@ -264,6 +264,7 @@ See [Vivraan/godot-lang-support](https://github.com/Vivraan/godot-lang-support).
 - [Maaack's Menus Template](https://github.com/Maaack/Godot-Menus-Template) - Template with a main menu, options menus, credits, and a scene loader.
 - [Maaack's Scene Loader](https://github.com/Maaack/Godot-Scene-Loader) - Scene loader that includes a loading screen, progress bar, and error handling.
 - [PixelPen](https://github.com/pixelpen-dev/pixelpen) - Pixel art drawing and animation: Common drawing tools such as selection, shape creation, mirroring, tiling, and animation features within the Godot editor.
+- [TerraBrush](https://github.com/spimort/TerraBrush) - Minimal Terrain heightmap editor that features sculpting, painting textures, LOD, and MultiZones.
 
 #### Godot 3.2+