Add some improved components docs.

docs/components.md

@@ -42,3 +42,18 @@ borrowed heavily from Svelte for this.
 ```
 
 
+If the component defines something like this:
+
+```
+<script>
+import Item from 'https://.../Item.component.xml';
+</script>
+
+<Item data={item}/>
+```
+
+Then the "import" node of the AST would be replaced by a function that
+provides the requested content (from Item.component.xml) like a macro.  When
+the `<Item/>` node is hit, it will be interpreted according to the imported
+component macro.
+