Add a better alt text (proposed by sajolida)

wiki/src/news/many_hands_make_tails.mdwn

@@ -46,8 +46,8 @@ one day.** If we could fund Tails entirely through donations by our
 users, this would make us independent from government grants.
 
 This image shows the evolution of Tails boots per day since 2013:
-[[!img boots_per_day.png link="no" class="margin center" alt="Tails
-boots per day 2013-2017"]]
+[[!img boots_per_day.png link="no" class="margin center" alt="Tails boots per
+day increasing steadily from 7000 in December 2013 to 22000 in October 2017"]]
 
 **But many of our users could actually get in trouble** if they donated to
 an anti-surveillance tool like Tails. So when donating to Tails you are