Add Zanata as an available translation platforms.

Adding Zanata to the list of suggested translation platforms.

Hi Ruben, I'm using angular-gettext for a small project I have and while reading the docs I noticed no mention of  Zanata. I happen to also be the lead for the Zanata developer team, and was wondering if you would accept this pull request to add it to the list; you know, just trying to get the word out :wink: