Add tarpaulin result file to `.gitignore`. #304

Job	Run time
Check default (ubuntu-latest, stable)	15s
Check all features (ubuntu-latest, stable)	22s
Check MSRV with all features	42s
Check benches with all features (ubuntu-latest, nightly)	21s
Check platform compatibility (ubuntu-latest, stable)	16s
Check CLI (ubuntu-latest, stable)	16s
Check platform compatibility (macOS-latest, stable)	35s
Check sync readme (ubuntu-latest, stable)	16s
Check platform compatibility (windows-latest, stable)	45s
Check, test, doc, format and lint with all features (ubuntu-latest, stable)	40s
Check, test, doc, format and lint with all features (ubuntu-latest, beta)	37s
Check, test, doc, format and lint with all features (ubuntu-latest, nightly)	46s
Coverage	2m 16s
	8m 7s

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