Add referral to third party translations in docs; closes #13094

Needs feedback on the phrasing, positioning and styling... pretty much everything.

Currently looks like this:
![bildschirmfoto 2014-05-14 um 00 34 53](https://cloud.githubusercontent.com/assets/1068978/2965111/12effe18-daef-11e3-8c5d-ed11dc5e7671.png)

Closes #13094.

/cc @juthilo @cvrebert @mdo @XhmikosR