Add note about symbols

Author: sindresorhus

readme.md

@@ -4,8 +4,8 @@
 
 ## Install
 
-```
-$ npm install filter-obj
+```sh
+npm install filter-obj
 ```
 
 ## Usage
@@ -27,6 +27,8 @@ const newObject2 = filterObject(object, ['bar']);
 
 ## API
 
+Symbol keys are not copied over to the new object.
+
 ### filterObject(source, filter)
 ### filterObject(source, includeKeys)