docs(readme): add crate downloads badge #338

Job	Run time
Formatting	15s
Check minimal supported rust version (MSRV).	2m 41s
Build and test (ubuntu, stable)	57s
Clippy (stable)	33s
Build and test (ubuntu, beta)	1m 3s
Clippy (beta)	29s
Build and test (ubuntu, nightly)	1m 6s
Build and test (macos, stable)	1m 31s
Clippy (nightly)	26s
Build and test (macos, beta)	1m 13s
Build and test (macos, nightly)	1m 24s
Build and test (windows, stable)	4m 0s
Build and test (windows, beta)	2m 31s
Build and test (windows, nightly)	4m 6s
	22m 15s

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