add a link to third party apps integrating in oc.

 That page needs improvements - I plan on working on it during the conference. @tomneedham @kelleybrooks @MariekevC what do you think, is this OK? Screenshot:
![oc documents2](https://cloud.githubusercontent.com/assets/551757/3909224/5b6fabca-230b-11e4-831b-c87524df8ce4.png)

(new is the link to third-party apps integrating in ownCloud)