fix #16 missing documentation: null/undefined as input value

content/docs/controlled-input-null-value.md

@@ -0,0 +1,19 @@
+---
+id: controlled-input-null-value
+title: Controlled input null value
+permalink: docs/controlled-input-null-value.html
+---
+
+Specifying the value prop on a [controlled component](/docs/forms.html) prevents the user from changing the input unless you desire so.
+
+You might have run into a problem where value is specified, but the input can still be changed without consent. In this case, you might have accidentally set value to undefined or null.
+
+For example, this code shows this phenomenon; after a second, the text becomes editable.
+```javascript
+ReactDOM.render(<input value="hi" />, mountNode);
+
+setTimeout(function() {
+  ReactDOM.render(<input value={null} />, mountNode);
+}, 1000);
+
+```