Add flake description #265

Job	Run time
Check flake.lock test (CEL condition)	8m 38s
Check flake.lock test (dirty 😈)	8m 18s
Nix and Rust checks	7m 36s
Test Rust	8m 59s
Check flake.lock test (clean 👼)	8m 16s
Check flake.lock test (dirty 😈 plus fail mode activated)	-1s
Build artifacts / build-artifacts (aarch64-darwin, macos-15, ARM64-macOS)	25m 27s
Build artifacts / build-artifacts (x86_64-linux, ubuntu-24.04, X64-Linux)	4m 3s
Build artifacts / build-artifacts (aarch64-linux, ubuntu-24.04-arm, ARM64-Linux)	7m 18s
Integration test for flake-checker-action (X64-Linux, ubuntu-24.04)	6s
Integration test for flake-checker-action (ARM64-Linux, ubuntu-24.04-arm)	4s
Integration test for flake-checker-action (ARM64-macOS, macos-15)	10s
	1h 18m 54s

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