Add migration guide for modified key export API

Signed-off-by: Hanno Becker <hanno.becker@arm.com>

docs/3.0-migration-guide.d/key-export.md

@@ -0,0 +1,28 @@
+SSL key export interface change
+-------------------------------
+
+This affects users of the SSL key export APIs:
+```
+    mbedtls_ssl_conf_export_keys_cb()
+    mbedtls_ssl_conf_export_keys_ext_cb()
+```
+
+The API `mbedtls_ssl_conf_export_keys_ext_cb()` has been removed,
+and the function type of key export callback passed to
+`mbedtls_ssl_conf_export_keys_cb()` has changed, as follows:
+- It no longer exports raw keys and IV.
+- A secret type parameter has been added to identify which key
+  is being exported. For TLS 1.2, only the master secret is
+  exported, but upcoming TLS 1.3 support will add other kinds of keys.
+
+For users which do not rely on raw keys and IV, adjusting to the new
+callback type should be straightforward - see the example programs
+programs/ssl/ssl_client2 and programs/ssl/ssl_server2 for callbacks
+for NSSKeylog, EAP-TLS and DTLS-SRTP.
+
+Users which require access to the raw keys used to secure application
+traffic may derive those by hand based on the master secret and the
+handshake transcript hashes which can be obtained from the raw data
+on the wire. Such users are also encouraged to reach out to the
+Mbed TLS team on the mailing list, to let the team know about their
+use case.