Added release notes for python-pillow#7882

Author: radarhere

docs/releasenotes/10.3.0.rst

@@ -67,6 +67,12 @@ Negative kmeans error
 When calling :py:meth:`~PIL.Image.Image.quantize`, a negative ``kmeans`` will now
 raise a :py:exc:`ValueError`, unless a palette is supplied to make the value redundant.
 
+Negative P1-P3 PPM value error
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If a P1-P3 PPM image contains a negative value, a :py:exc:`ValueError` will now be
+raised.
+
 API Additions
 =============