Add changelog

Signed-off-by: Felix Conway <felix.conway@arm.com>
diff --git a/ChangeLog.d/pem-integer-underflow.txt b/ChangeLog.d/pem-integer-underflow.txt
new file mode 100644
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+++ b/ChangeLog.d/pem-integer-underflow.txt
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+Security
+   * Fix an integer underflow that could occur when parsing malformed PEM
+     keys, which could be used by an attacker capable of feeding encrypted
+     PEM keys to a user. This could cause a crash or information disclosure.
+     Found and reported by Linh Le and Ngan Nguyen from Calif.