Add stbimage tutorial.

docs/ReFuel.StbImage.md

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+# ReFuel.StbImage
+StbImage is a very common library that has been developed mainly by Sean Barett.
+It is one of the easiest to use image loading libraries on the planet
+especially for C and C++ users. It is relatively simple to load up and use in
+C#, however the C# build system makes it a bit difficult to manage native
+dependencies. Therefore this package makes it easy for you and everyone else
+who needs a cross platform way to load images.
+
+Intended OS targets are:
+* Windows (x86 and x64)
+* Linux (arm32, arm64, x86 and x64)
+* MacOS (arm64, x64)
+
+## Usage
+```cs
+using ReFuel.Stb;
+
+// Simply load an image from a Stream or Span<T> (T: unmanaged) source.
+using StbImage image = StbImage.Load(source);
+
+Console.WriteLine("{0}x{1}", image.Width, image.Height);
+// StbImage.ImagePointer is a pointer to the image data.
+// Pass the image to an unsafe library as is (e.g. OpenGL), or cast it to a span. Currently there is no "safe" way to do this.
+
+// OpenGL example
+GL.TexImage2D(TextureTarget.Texture2D, 0, PixelInternalFormat.Rgba, image.Width, image.Height, 0, StbiFormatToGLFormat(image.Format, image.IsFloat ? PixelFormat.Float : PixelFormat.UnsignedByte), image.ImagePointer);
+
+// C# example
+struct Pixel
+{
+    int R, G, B, A;
+}
+
+Span<Pixel> pixels;
+unsafe
+{
+    pixels = new Span<Pixel>(image.ImagePointer, image.Width * image.Height);
+}
+```
+
+### Global Options
+| Option | Description |
+|--------|-------------|
+| `bool StbImage.FlipVerticallyOnLoad` | Flips the image vertically after loading. Practical for uses where a right handed coordinate system is used (e.g. OpenGL) |
+| `bool UnpremultiplyOnLoad` | Applies to Apple iPhone PNG images, where the image is sometimes encoded with premultiplied alpha. Reverses this premultiplication. |
+
+## Calling Stbi directly.
+If you wish to call the native Stbi library, call functions in the static class `Stbi`. See `stb_image.h` documentation [online](https://github.com/nothings/stb/blob/master/stb_image.h).

docs/getting-started.md

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-# Getting Started

docs/introduction.md

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-# Introduction

docs/toc.yml

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-- name: Introduction
+- name: ReFuel.StbImage Guide
-  href: introduction.md
+  href: ReFuel.StbImage.md
-- name: Getting Started
-  href: getting-started.md