Add information on JSX element keys (#6751)

docs/02.3-jsx-gotchas.md

@@ -40,10 +40,10 @@ A safer alternative is to find the [unicode number corresponding to the entity](
 <div>{'First ' + String.fromCharCode(183) + ' Second'}</div>
 ```
 
-You can use mixed arrays with strings and JSX elements.
+You can use mixed arrays with strings and JSX elements. Each JSX element in the array needs a unique key.
 
 ```javascript
-<div>{['First ', <span>&middot;</span>, ' Second']}</div>
+<div>{['First ', <span key="middot">&middot;</span>, ' Second']}</div>
 ```
 
 As a last resort, you always have the ability to [insert raw HTML](/react/tips/dangerously-set-inner-html.html).