update README

Author: jcubic

README.md

@@ -120,7 +120,7 @@ sysend object:
 | `proxy(<urls>)` | create iframe proxy for different domain, the target domain/URL should have [proxy.html](https://github.com/jcubic/sysend.js/blob/master/proxy.html)<br> file. If url domain is the same as page domain, it's ignored. So you can put both proxy calls on both | url - string | 1.3.0 |
 | `serializer(to_string, from_string)` | add serializer and deserializer functions | both arguments are functions (data: any) => string | 1.4.0 |
 | `emit(name, [, object])` | same as `broadcast()` but also invoke the even on same page | name - string - The name of the event<br>object - optional any data | 1.5.0 |
-| `post(<window_id>, [, object])` | send any data to other window | window_id - string of the target window<br>object - any data | 1.6.0 |
+| `post(<window_id>, [, object])` | send any data to other window | window_id - string of the target window (use `'primary'` to send to primary window)<br>object - any data | 1.6.0 / `'primary'` target 1.14.0 |
 | `list()` | returns a Promise of objects `{id:<UUID>, primary}` for other windows, you can use those to send a message with `post()` | NA | 1.6.0 |
 | `track(event, callback)` | track inter window communication events  | event - any of the strings: `"open"`, `"close"`, `"primary"`, <br>`"secondary"`, `"message"`<br>callback - different function depend on the event:<br>* `"message"` - `{data, origin}` - where data is anything the `post()` sends, and origin is `id` of the sender.<br>* `"open"` - `{count, primary, id}` when new window/tab is opened<br>* `"close"` - `{count, primary, id, self}` when window/tab is closed<br>* `"primary"` and `"secondary"` function has no arguments and is called when window/tab become secondary or primary.<br>* `"ready"` - event when tracking is ready. | 1.6.0 except `ready` - 1.10.0 |
 | `untrack(event [,callback])` | remove sigle event listener all all listeners for a given event | event - any of the strings `'open'`, `'close'`, `'primary'`, `'secondary'`, or `'message'`. | 1.6.0 |