Add info about 3.5 release

When the release is ready we need to add some infos about the changes:

• Outgoing Autocrypt Headers schleuder!299 (merged)
• deliver_selfsent schleuder!302 (merged)
• Visual separator schleuder!305 (merged)
• do not recognize sudo message as automated schleuder!307 (merged)
• improved wording about how many keys where found and usable schleuder!309 (merged)
• handle decryption problems more gracefully schleuder!310 (merged)
• x-attach-listkey works now schleuder!311 (merged)
• list-id header in notifications sent to admins schleuder!312 (merged)
• more proper encoding handling schleuder!325 (merged)