Add Changelog entry

Add Changelog entry for changes made in this PR.

Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>
diff --git a/ChangeLog.d/x509_ext_types_accessor.txt b/ChangeLog.d/x509_ext_types_accessor.txt
new file mode 100644
index 0000000..13a3152
--- /dev/null
+++ b/ChangeLog.d/x509_ext_types_accessor.txt
@@ -0,0 +1,3 @@
+Features
+   * Add the function mbedtls_x509_crt_has_ext_type() to access the ext types
+     field within mbedtls_x509_crt context, as requested in #5585.