Add changelog entry

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/ChangeLog.d/alignment-perf.txt b/ChangeLog.d/alignment-perf.txt
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+++ b/ChangeLog.d/alignment-perf.txt
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+Features
+   * Improvements to the code used to access unaligned and byte-swapped memory
+     reduce code size and improve performance (depending on compiler and target
+     architecture).