Update to warn against Function constructor

Added section 7.9 to warn against using the Function creator, as it opens similar vulnerabilities as eval.
Originally suggested in pull request #395.

No other lines were edited or removed.

README.md

@@ -567,6 +567,14 @@
   count();  // 3
   ```
 
+- [7.9](#7.9) <a name='7.9'></a> Never use the Function constructor to create a new function.
+
+  > Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.
+
+  ```javascript
+  // bad
+  var add = new Function("a", "b", "return a + b");
+  ```
 
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