Add a link to the upgrade notes

Author: babolivier

CHANGES.md

@@ -3,6 +3,8 @@ Synapse 1.45.0rc1 (2021-10-12)
 
 **Note:** We are aware of [a performance issue](https://github.com/matrix-org/synapse/issues/11049) introduced in Synapse v1.44.0, as well as [a bug](https://github.com/matrix-org/synapse/issues/11025) with the user directory when using application services. While this release candidate doesn't fix either of those issues, a second release candidate is expected to come out in a few days to address them.
 
+Media storage providers module that read from Synapse's configuration need changes as of this version, see the [upgrade notes](https://matrix-org.github.io/synapse/develop/upgrade#upgrading-to-v1450) for more informations.
+
 Features
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