Add symbol as a primitive type

- Also add note that symbols cannot by polyfilled and should not be used in environments that don't support them

README.md

@@ -68,6 +68,7 @@ Other Style Guides
     - `boolean`
     - `null`
     - `undefined`
+    - `symbol`
 
     ```javascript
     const foo = 1;
@@ -78,6 +79,8 @@ Other Style Guides
     console.log(foo, bar); // => 1, 9
     ```
 
+    - Symbols cannot be faithfully polyfilled, so they should not be used when targeting browsers/environments that don't support them natively.
+
   <a name="types--complex"></a><a name="1.2"></a>
   - [1.2](#types--complex)  **Complex**: When you access a complex type you work on a reference to its value.