Add changelog for 0.12 release #4298

Job	Run time
Build docs	1m 6s
Check minimal versions	23s
Test (macOS)	4m 36s
Test (nightly)	5m 24s
MSRV	0s
Examples	5m 16s
Test (stable)	6m 40s
Test (beta)	5m 37s
Test (Windows)	7m 56s
Test (native-tls)	4m 34s
Test (no default features)	59s
Test (no cache)	7m 3s
Test (no gateway)	2m 29s
Test (unstable Discord API features)	2m 10s
Test (simd-json)	6m 43s
Test (chrono, chrono)	55s
Test (builder without model, builder, true)	45s
Test (unstable Discord API (no default features), unstable_discord_api, true)	33s
	1h 3m 9s

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