docs: missing changelog or release note for v2.8.0

[gaeljw]

Hello,

Thanks for releasing the long awaited 2.8.0 :)

I haven't seen any changelog or release note though unless I'm not looking in the right place.

[mkurz]

/cc @cchantep

[gaeljw]

FWIW, I've come up with the following list of "main" changes from the commit list:

• Fix Streaming with Akka-Anorm generates a materialized value as Future that may never complete #545
• Support Pekko Streaming (Add PekkoStream #572)