CHANGES: warn prominently about gRPC transition being a breaking change

Users may not read the release notes up to the third section.
Add a prominent note early on.

Signed-off-by: Leonard Göhrs <[email protected]>

Author: hnez

CHANGES.rst

@@ -14,6 +14,11 @@ That's why labgrid moves to gRPC with this release. gRPC is a well maintained
 RPC framework with a lot of users. As a side effect, the message transfer is
 more performant and the import times are shorter.
 
+This is a **breaking change**. Version 25.0 exporters / coordinators / clients can
+not communicate with Version 24.0 and earlier exporters / coordinators / clients.
+All exporters, coordinators and clients in a deployment have to be updated at
+the same time. See the "Breaking changes in 25.0" section below for more information.
+
 New Features in 25.0
 ~~~~~~~~~~~~~~~~~~~~
 - All components can be installed into the same virtualenv again.