Add info about change in default flow control value (#673)

guides/upgrading/v1.0.0.md

@@ -90,6 +90,29 @@ use Membrane.Filter
 
 Check `t:Membrane.Pad.element_spec/0` for details.
 
+### Adjust to change in default `flow_control` (aka. `mode`/`demand_mode`) value.
+
+Now, if definition of pad in element does specify type of 
+
+Now, the default value of `flow_control` option in `def_input_pad` and `def_output_pad` in Elements is `:auto`. Until `v0.12.9`, pads that sepcified neither `flow_control`, `mode` nor `demand_mode` explicitly would have `:manual` `flow_control`. Therefore, `:manual` pads that haven't specified `flow_control` value, now have to do it explicitly.
+
+```diff
+- def_output_pad :output, accepted_format: %MyStruct{field: :value}
++ def_output_pad :output, accepted_format: %MyStruct{field: :value}, flow_control: :manual
+```
+
+```diff
+- def_input_pad :input, accepted_format: %MyStruct{field: :value}, demand_unit: :buffers
++ def_input_pad :input, accepted_format: %MyStruct{field: :value}, demand_unit: :buffers, flow_control: :manual
+```
+
+Moreover, you can remove `flow_control: :auto` from the pad definitions, if you want to.
+
+```diff
+- def_input_pad :input, accepted_format: %MyStruct{field: :value}, flow_control: :auto
++ def_input_pad :input, accepted_format: %MyStruct{field: :value}
+```
+
 ### Remove `mode` and `demand_unit` from pads definitions in bins
 
 ```diff