Add changelog

Signed-off-by: Chien Wong <m@xv97.com>
diff --git a/ChangeLog.d/improve-doc-on-ecp-curve-optimized-representation.txt b/ChangeLog.d/improve-doc-on-ecp-curve-optimized-representation.txt
new file mode 100644
index 0000000..8fdc588
--- /dev/null
+++ b/ChangeLog.d/improve-doc-on-ecp-curve-optimized-representation.txt
@@ -0,0 +1,3 @@
+Features
+   * The documentation of mbedtls_ecp_group now describes the optimized
+     representation of A for some curves. Fixes #8045.