Add a section to the guide about the Paint example

guide/src/web-sys/examples/paint.md

@@ -0,0 +1,24 @@
+# Paint
+
+A simple painting program.
+
+[See the full source at
+`wasm-bindgen/examples/paint`.](https://github.com/rustwasm/wasm-bindgen/tree/master/examples/paint)
+
+## `Cargo.toml`
+
+The `Cargo.toml` enables features necessary to work with the DOM, events and
+2D canvas.
+
+```toml
+{{#include ../../../../examples/paint/Cargo.toml}}
+```
+
+## `src/lib.rs`
+
+Creates the `<canvas>` element, applies a CSS style to it, adds it to the document,
+get a 2D rendering context and adds listeners for mouse events.
+
+```rust
+{{#include ../../../../examples/paint/src/lib.rs}}
+```