docs(changelog): add the #1557 fragment

changelog.d/1557-cockpit-ship-bridge.md

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+- Ride the selected AIS vessel from its bridge: an eye height by vessel type,
+  dead reckoning between fixes, a bridge HUD with a 3–24 NM radar, look-around
+  with the arrow keys, and a waterline read from the scene so river boats sit on
+  the river rather than at sea level (Denis Groshev, @ZloyS, #1557).