Add release notes for v0.3.15. #53

Job	Run time
Verify Docs Build / Verify RTD Docs	3m 18s
CI / Pre-commit checks / Pre-commit Checks	47s
CI / Check towncrier / Towncrier Checks	26s
CI / Python package / Create Python Package	58s
CI / Unit tests (macos, 3.8)	1m 59s
CI / Unit tests (macos, 3.9)	2m 6s
CI / Unit tests (macos, 3.10)	2m 3s
CI / Unit tests (macos, 3.11)	1m 58s
CI / Unit tests (macos, 3.12-dev)	2m 9s
CI / Unit tests (ubuntu, 3.8)	1m 22s
CI / Unit tests (ubuntu, 3.9)	1m 22s
CI / Unit tests (ubuntu, 3.10)	1m 32s
CI / Unit tests (ubuntu, 3.11)	1m 18s
CI / Unit tests (ubuntu, 3.12-dev)	2m 16s
CI / Unit tests (windows, 3.8)	2m 6s
CI / Unit tests (windows, 3.9)	2m 8s
CI / Unit tests (windows, 3.10)	2m 6s
CI / Unit tests (windows, 3.11)	2m 7s
CI / Unit tests (windows, 3.12-dev)	2m 9s
CI / Unit tests (windows, winstore3.11, false)	2m 7s
CI / Project coverage	1s
CI / Build app	1s
Create Release	1s
	36m 20s

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