Added key object spread example

Spreaded key is discouraged and currently triggering the rule. Added some clarification to the doc.

Resolves jsx-eslint#1753.

docs/rules/jsx-key.md

@@ -11,14 +11,20 @@ The following patterns are considered warnings:
 [<Hello />, <Hello />, <Hello />];
 
 data.map(x => <Hello>{x}</Hello>);
+
+<Hello {...{ key: id, id, caption }} />
 ```
 
+In the last example the key is being spread, which is currently possible, but discouraged in favor of the statically provided key.
+
 The following patterns are **not** considered warnings:
 
 ```jsx
 [<Hello key="first" />, <Hello key="second" />, <Hello key="third" />];
 
 data.map((x, i) => <Hello key={i}>{x}</Hello>);
+
+<Hello key={id} {...{ id, caption }} />
 ```
 
 ## When not to use