add unison to file synchronization

Unison is a very nice file synchronization program licensed under GPL.

README.md

@@ -484,6 +484,7 @@ Some [Groupware](#groupware) solutions also feature file sharing and synchroniza
   * [Z-Push](https://z-push.org/) - An implementation of Microsoft’s [ActiveSync](https://en.wikipedia.org/wiki/ActiveSync) protocol
   * [ownCloud](https://owncloud.org/) - All-in-one solution for saving, synchronizing, viewing, editing and sharing files, calendars, address books and more - `AGPLv3` `PHP`
   * [Pydio](https://pydio.com/) - Turn any web server into a powerful file management system and an alternative to mainstream cloud storage providers.
+  * [Unison](https://www.cis.upenn.edu/~bcpierce/unison/) - Unison is a file-synchronization tool for OSX, Unix, and Windows.
 
 
 #### Peer-to-peer filesharing