DH: add CHANGES entry listing the deprecated DH functions.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11024)

CHANGES

@@ -74,6 +74,29 @@
      programs respectively.
      [Paul Dale]
 
+  *) All of the low level DH functions have been deprecated including:
+
+     DH_OpenSSL, DH_set_default_method, DH_get_default_method, DH_set_method,
+     DH_new_method, DH_bits, DH_size, DH_security_bits, DH_get_ex_new_index,
+     DH_set_ex_data, DH_get_ex_data, DH_generate_parameters_ex,
+     DH_check_params_ex, DH_check_ex, DH_check_pub_key_ex,
+     DH_check, DH_check_pub_key, DH_generate_key, DH_compute_key,
+     DH_compute_key_padded, DHparams_print_fp, DHparams_print, DH_get_nid,
+     DH_KDF_X9_42, DH_get0_engine, DH_get_length, DH_set_length, DH_meth_new,
+     DH_meth_free, DH_meth_dup, DH_meth_get0_name, DH_meth_set1_name,
+     DH_meth_get_flags, DH_meth_set_flags, DH_meth_get0_app_data,
+     DH_meth_set0_app_data, DH_meth_get_generate_key,
+     DH_meth_set_generate_key, DH_meth_get_compute_key,
+     DH_meth_set_compute_key, DH_meth_get_bn_mod_exp,
+     DH_meth_set_bn_mod_exp, DH_meth_get_init, DH_meth_set_init,
+     DH_meth_get_finish, DH_meth_set_finish, DH_meth_get_generate_params
+     and DH_meth_set_generate_params.
+
+     Use of these low level functions has been informally discouraged for a long
+     time.  Instead applications should use L<EVP_PKEY_derive_init(3)>
+     and L<EVP_PKEY_derive(3)>.
+     [Paul Dale]
+
   *) All of the low level DSA functions have been deprecated including:
 
      DSA_do_sign, DSA_do_verify, DSA_OpenSSL, DSA_set_default_method,