Create NOTICE.md

Author: soupglasses

NOTICE.md

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+Content under `nixos/` has sections inspired by [Numtide's SrvOS project](https://github.com/numtide/srvos).
+- Copyright (c) 2023 Numtide
+- https://opensource.org/license/mit/
+
+Image under `docs/media/the-declarative-trinity.png`.
+- Copyright (c) 2021 reddit.com/u/SEbbaDK
+- https://creativecommons.org/licenses/by-sa/4.0/