Add initial changelog for 1.16

[cdce8p]

Create changelog entries for

• Support properties with setter type different from getter type #18510
• Add option to selectively disable deprecation warnings #18641

[Hnasar]

Great idea to keep a running changelog, so there's not a large effort to write it in retrospect at the end!

[hauntsaninja]

Looks good, thank you! For 1.15 Jukka had requested to use title case for the headings