Add changelog for 0.12 release #4295

Job	Run time
Test (stable)	5m 0s
Test (beta)	5m 44s
Build docs	1m 6s
Test (Windows)	6m 58s
Examples	6m 19s
Test (native-tls)	4m 28s
Test (macOS)	5m 13s
Check minimal versions	26s
Test (no default features)	1m 2s
MSRV	0s
Test (no cache)	5m 28s
Test (nightly)	6m 17s
Test (no gateway)	2m 38s
Test (unstable Discord API features)	1m 34s
Test (simd-json)	6m 21s
Test (chrono, chrono)	58s
Test (builder without model, builder, true)	47s
Test (unstable Discord API (no default features), unstable_discord_api, true)	41s
	1h 1m 0s

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