Add changelog entry for #550

Author: michaelbaisch

CHANGES.rst

@@ -7,6 +7,10 @@
 - Fix ``Observer`` not having longtitude, latitude, and elevation parameters
   as class attributes. They are now properties calculated from the ``location``.
 
+- Fix ``block already occupied`` error in ``PriorityScheduler`` by adjusting
+  the condition for filled slots in ``_get_filled_indices()``. This was primarily
+  an issue for blocks with ``duration`` <= ``time_resolution``. [#550]
+
 0.8 (2021-01-26)
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