Add Image! docs to book (#652)

docs/src/SUMMARY.md

@@ -10,5 +10,6 @@
 - [Features]()
     - [Attribute syntax](./attributes.md)
     - [Inline Assembly](./inline-asm.md)
+    - [Image type syntax](./image.md)
 - [RFCs]()
     - [001. Resource Binding Syntax](./rfcs/001-resource-binding-syntax.md)

docs/src/image.md

@@ -0,0 +1,26 @@
+# Image type syntax
+
+There are a huge number of combinations of image types in SPIR-V. They are represented by a const
+generic type called `spirv_std::image::Image`, however, specifying the generic parameters of this
+type is incredibly tedious, so a wrapper macro, `spirv_std::Image!` can be used to write the type
+instead.
+
+The specific syntax and meaning of the arguments to the `Image!` macro can be found in
+[rustdoc](https://embarkstudios.github.io/rust-gpu/api/spirv_std/macro.Image.html).
+
+Some type aliases for common image formats can be found in the
+[`spirv_std::image`](https://embarkstudios.github.io/rust-gpu/api/spirv_std/image/index.html)
+module. For example, `Image2d` is a very commonly used type, corresponding to `texture2D` in GLSL,
+and is likely what you want if you want a regular old sampled texture.
+
+```rust,no_run
+type Image2d = Image!(2D, type=f32, sampled);
+```
+
+Note that the `const-generics` cargo feature in spirv-std must be enabled to use the `Image!` macro
+at the moment.  This will likely change in the near future.
+
+```toml
+[dependencies]
+spirv-std = { ..., features = ["const-generics"] }
+```