CHANGES: add link to the gRPC website

For someone without experience in the field of RPC frameworks the
words "the first release using gRPC instead of crossbar/autobahn"
may not carry a lot of meaning.

Adding a link may reduce some confusion.

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

Author: hnez

CHANGES.rst

@@ -2,7 +2,7 @@ Release 25.0 (Unreleased)
 -------------------------
 As announced
 `before <https://github.com/labgrid-project/labgrid/discussions/1467#discussioncomment-10314852>`_,
-this is the first release using gRPC instead of crossbar/autobahn for
+this is the first release using `gRPC <https://grpc.io/>`_ instead of crossbar/autobahn for
 communication between client/exporter and coordinator.
 
 Crossbar/autobahn are unfortunately not very well maintained anymore. The