Add ChangeLog and migration guide entries

Merge part of the RSA entries into this one, as I think it's easier for
users to have all similar changes in one place regardless of whether
they were introduce in the same PR or not.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/ChangeLog.d/mandatory-rng-param.txt b/ChangeLog.d/mandatory-rng-param.txt
new file mode 100644
index 0000000..4e04248
--- /dev/null
+++ b/ChangeLog.d/mandatory-rng-param.txt
@@ -0,0 +1,12 @@
+API changes
+   * For all functions that take an RNG parameter, this parameter is now
+     mandatory (that is, NULL is not an acceptable value). Functions which
+     previously accepted NULL and now reject it are: the X.509 CRT and CSR
+     writing functions; the PK sign and decrypt function; the RSA encrypt,
+     decrypt, sign and private functions; the function in DHM and ECDH that
+     compute the share secret; the scalar multiplication functions in ECP.
+   * The following functions now require an RNG parameter:
+     mbedtls_ecp_check_pub_priv(), mbedtls_pk_check_pair(),
+     mbedtls_pk_parse_key(), mbedtls_pk_parse_keyfile().
+   * The configuration option MBEDTLS_ECP_NO_INTERNAL_RNG has been removed as
+     it no longer had any effect.
diff --git a/ChangeLog.d/remove-rsa-mode-parameter.txt b/ChangeLog.d/remove-rsa-mode-parameter.txt
index 854dda3..2590d3a 100644
--- a/ChangeLog.d/remove-rsa-mode-parameter.txt
+++ b/ChangeLog.d/remove-rsa-mode-parameter.txt
@@ -6,4 +6,3 @@
      decryption functions now always use the private key and verification and
      encryption use the public key. Verification functions also no longer have
      RNG parameters.
-   * The RNG is now mandatory for all private-key RSA operations.