Add changes file for 5.9.0

Listing important changes and bug-fixes.

Done-with: Thiago Macieira <[email protected]>
Done-with: Oswald Buddenhagen <[email protected]>
Done-with: Shawn Rutledge <[email protected]>
Done-with: Timur Pocheptsov <[email protected]>
Change-Id: I147d6986c565b5adb41a6a3a18a5f1e82cfe9b64
Reviewed-by: Lars Knoll <[email protected]>
Reviewed-by: Shawn Rutledge <[email protected]>
Reviewed-by: Oswald Buddenhagen <[email protected]>