add changelog

Co-Authored-By: Jernej Kos <[email protected]>

.changelog/2699.feature.md

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+go/extra/stats: Availability ranking for next Quest phase
+
+A new availability score will take into account more than the number of
+block signatures alone.
+This introduces the mechanism to compute a score and print the
+rankings based on that.
+This also implements a provisional scoring formula.