clarify prepend in readme

Author: djih

README.md

@@ -120,7 +120,7 @@ The SDK supports the operations `set`, `setOnce`, `unset`, and `add` on individu
     amplitude.identify(identify);
     ```
 
-6. `prepend`: this will prepend a value or values to a user property. If the user property does not have a value set yet, it will be initialized to an empty list before the new values are prepended. If the user property has an existing value and it is not a list, it will be converted into a list with the new value prepended.
+6. `prepend`: this will prepend a value or values to a user property. Prepend means inserting the value(s) at the front of a given list. If the user property does not have a value set yet, it will be initialized to an empty list before the new values are prepended. If the user property has an existing value and it is not a list, it will be converted into a list with the new value prepended.
 
     ```javascript
     var identify = new amplitude.Identify().prepend('ab-tests', 'new-user-test').prepend('some_list', [1, 2, 3, 4, 'values']);