Add migration guide
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/docs/3.0-migration-guide.d/max-record-payload-api.md b/docs/3.0-migration-guide.d/max-record-payload-api.md
new file mode 100644
index 0000000..0b34915
--- /dev/null
+++ b/docs/3.0-migration-guide.d/max-record-payload-api.md
@@ -0,0 +1,11 @@
+Remove MaximumFragmentLength (MFL) query API
+-----------------------------------------------------------------
+
+This affects users which use the MFL query APIs
+`mbedtls_ssl_get_{input,output}_max_frag_len()` to
+infer upper bounds on the plaintext size of incoming and
+outgoing record.
+
+Users should switch to `mbedtls_ssl_get_max_{in,out}_record_payload()`
+instead, which also provides such upper bounds but takes more factors
+than just the MFL configuration into account.