Add contributing doc for eyre #270

Job	Run time
Miri	1m 30s
Rustfmt (stable)	11s
Clippy (stable)	28s
Check (stable)	31s
Test Suite (stable)	47s
Test Suite (ubuntu-latest)	52s
Test Suite (stable, --no-default-features)	1m 1s
Test Suite	5m 29s
Test Suite (windows-latest)	2m 50s
Test Suite (stable, --features track-caller)	1m 15s
Test Suite (--no-default-features)	6m 5s
Test Suite (macOS-latest)	45s
Test Suite (stable, --features pyo3)	57s
Test Suite (--features track-caller)	6m 8s
Test Suite (stable, --features auto-install)	54s
Test Suite (--features auto-install)	6m 19s
Test Suite (stable, --all-features)	53s
Test Suite (beta)	52s
Test Suite (beta, --no-default-features)	1m 3s
Test Suite (beta, --features track-caller)	56s
Test Suite (beta, --features pyo3)	56s
Test Suite (beta, --features auto-install)	53s
Test Suite (beta, --all-features)	52s
Test Suite (nightly)	1m 11s
Test Suite (nightly, --no-default-features)	3m 6s
Test Suite (nightly, --features track-caller)	1m 5s
Test Suite (nightly, --features pyo3)	1m 27s
Test Suite (nightly, --features auto-install)	3m 3s
Test Suite (nightly, --all-features)	1m 11s
	53m 30s

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