docs: Create readme.

README.md

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+# Powerspheres
+
+## Features
+
+- Noise generation
+    - Perlin noise and derivatives:
+    - Fractal noise and curl noise
+    - Seamlessly tiling noise
+- Dynamic grass generation in geometry shaders
+    - Shape controllable by various parameters: no. of segments, height,
+    width, curve.
+    - Quasirandom placement controllable by density slider, one-channel
+    mask, and subdivision into grids
+    - Reacts to player movement and wind
+    - Supports LOD simplification
+- Leaf-like particle system
+    - Can be picked up by wind
+    - Rendered billboards align to trajectory
+- Wind system
+    - Supports arbitrary wind data fed in from a texture
+    - A few controllable parameters such as wind speed and frequency
+    - Future extension: write modifications to wind texture based on player
+    interaction
+- URP-compatible shaders
+    - Distance-based shading / material swapping
+    - Simple Cook-Torrance shading using Lambertian diffuse and Blinn-Phong
+    NDF and combined fresnel-visibility approximation for specular
+    - Default URP GI shading for indirect
+    - Post-processing effects in compute shader
+
+## Demo controls
+
+### Mouse+keyboard
+- Right/Left mouse buttons: Pick up or drop sphere for corresponding side
+- WASD / Arrow keys: Movement
+- Mouse movement: Look
+- Space: Jump
+
+### Controller (Xbox)
+- RT/LT: Pick up / drop
+- Dpad: Movement
+- Right stick: Look
+- Y: Jump
+
+------
+
+### Digital art concept
+
+#### Title & concept
+Hidden miniature worlds
+
+#### Explanation of concept
+A colorless, formless world is gradually revealed
+by interacting with miniature spheres found scattered around. Each one, a
+tiny world unto itself, when released, will fill the outer world with its
+influence.
+
+It is possible to control each sphere independently and observe the
+combinations of effects possible under their influence. However, to suit
+traditional control schemes, this demo only allows moving two each time.