Add upgrade docs

Author: raphaelm

docs/index.rst

@@ -20,6 +20,7 @@ Library user documentation content
    transfers
    debits
    tested
+   upgrading_4_5
    upgrading_3_4
    upgrading_2_3
    upgrading_1_2

docs/upgrading_4_5.rst

@@ -0,0 +1,12 @@
+Upgrading from python-fints 4.x to 5.x
+======================================
+
+Release 5.0 of this library was made to introduce breaking changes:
+
+* When your bank no longer supports fetching MT940 bank statements, `get_transactions()` will transparently fall back
+  to retrieve XML statements and convert them to dictionaries. While we tried to keep the most important fields in
+  place, this will cause different output. Please verify that the new output does not cause issues in your application.
+
+* When sending a credit transfer, you might now receive a `NeedVOPResponse` instead of a `NeedTANResponse` if there is
+  no exact match of payer names. You can proceed with `approve_vop_response()` and will then receive your
+  `NeedTANResponse` as usual.