docs: added AlternativeTo rating badge

Author: ErikBjare

stats.pug

@@ -75,6 +75,12 @@ div.row.text-center
       a(href="https://aur.archlinux.org/packages/activitywatch-bin/")
         img(src="https://img.shields.io/aur/votes/activitywatch-bin?logo=arch-linux")
 
+  div.col-sm-3
+    h6 AlternativeTo
+    p
+      a(href="https://alternativeto.net/software/activitywatch/about/")
+        img(src="https://img.shields.io/badge/rating-4.7%2F5%20(124)-green")
+
   div.col-sm-3
     h6 Play Store
     p