Document the remaining constraints on ALT context types

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/alt-context-relaxation.txt b/ChangeLog.d/alt-context-relaxation.txt
new file mode 100644
index 0000000..10fd476
--- /dev/null
+++ b/ChangeLog.d/alt-context-relaxation.txt
@@ -0,0 +1,6 @@
+Features
+   * Alternative implementations of the AES, DHM, ECJPAKE, ECP, RSA and timing
+     modules had undocumented constraints on their context types. These
+     constraints have been relaxed.
+     See docs/architecture/alternative-implementations.md for the remaining
+     constraints.