Changelog entry for adding mbedtls_mpi_random()

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/mpi_random.txt b/ChangeLog.d/mpi_random.txt
new file mode 100644
index 0000000..9e6a416
--- /dev/null
+++ b/ChangeLog.d/mpi_random.txt
@@ -0,0 +1,3 @@
+Features
+   * The new function mbedtls_mpi_random() generates a random value in a
+     given range uniformly.