docs: added ProductHunt launch to timeline events

Author: ErikBjare

_data/milestones.yml

@@ -59,6 +59,10 @@
 - title: "100 active Android installs"
   date: "2019-12-27"
 
+- title: "Launched on Product Hunt"
+  date: "2019-7-3"
+  url: "https://www.producthunt.com/posts/activitywatch"
+
 - title: "ActivityWatch for Android announced \"usable\""
   date: "2019-04-19"  # approximate
   url: "https://twitter.com/ActivityWatchIt/status/1119258099334795264"