docs: add documentaion for keys method

Author: ahmad-kashkoush

lib/node_modules/@stdlib/array/fixed-endian-factory/README.md

@@ -543,6 +543,28 @@ var v = arr.get( 100 );
 // returns undefined
 ```
 
+<a name="method-keys"></a>
+
+#### TypedArrayFE.prototype.keys()
+
+Returns an iterator for iterating over each index key in a typed array.
+
+```javascript
+var Float64ArrayFE = fixedEndianFactory( 'float64' );
+
+var arr = new Float64ArrayFE( 'little-endian', 2 );
+var iter = arr.keys();
+
+var v = iter.next().value;
+// returns 0
+
+v = iter.next().value;
+// returns 1
+
+var bool = iter.next().done;
+// returns true
+```
+
 <a name="method-index-of"></a>
 
 #### TypedArrayFE.prototype.indexOf( searchElement\[, fromIndex] )