fix(docs) Add custom attributes and Snippets documentation

markdown/dev/reference/snippets/en.md

@@ -2,7 +2,28 @@
 title: Snippets
 ---
 
-A Snippet is a reuseable bit of markup for your pattern. They are provided by
-plugins. Below is an overview of all the different snippets we maintain.
+A Snippet is a reusable bit of markup that draws graphic elements on patterns.
+Snippets are provided by [plugins](/reference/plugins) and are implemented as
+[<defs> elements](https://developer.mozilla.org/en-US/docs/Web/SVG/Element/defs)
+in the rendered SVG document.
+
+## Signature
+
+To place a Snippet at a given point:
+```js
+const snippet = new Snippet(String 'snippet name', Point anchor)
+```
+
+<Related>
+See the [Snippet API](/reference/api/snippet)
+for more information about how to use Snippets in code.
+
+See [Using Attributes](/howtos/code/attributes)
+for information about custom Attributes that can be used with Snippets.
+</Related>
+
+## List of snippets
+
+Below are all the different snippets we maintain.
 
 <ReadMore list />