add placeholder reference

docs/reference/components/placeholder.md

@@ -0,0 +1,55 @@
+
+# `<Placeholder>`
+
+```js
+import { Placeholder } from 'slate'
+```
+
+A simple component that adds a placeholder to a node. It encapsulates all of the Slate-related logic that determines when to render the placeholder, so you don't have to think about it.
+
+- [Properties](#properties)
+  - [`children`](#children-any)
+  - [`className`](#className-string)
+  - [`node`](#node-node)
+  - [`parent`](#parent-node)
+  - [`state`](#state-state)
+  - [`style`](#style-object)
+
+
+## Properties
+
+```js
+<Placeholder
+  className={String}
+  node={Node}
+  parent={Node}
+  state={State}
+  style={Object}
+>
+  {children}
+</Placeholder>
+```
+
+#### `children: Any`
+
+React child elements to render inside the placeholder `<span>` element.
+
+#### `className: String`
+
+An optional class name string to add to the placeholder `<span>` element.
+
+#### `node: Node`
+
+The node to render the placeholder element on top of. The placeholder is positioned absolutely, covering the entire node.
+
+#### `parent: Node`
+
+The node to check for non-empty content, to determine whether the placeholder should be shown or not.
+
+#### `state: State`
+
+The current state of the editor.
+
+#### `style: Object`
+
+An optional dictionary of styles to pass to the placeholder `<span>` element.