Rust

README: clarify just how encouraged contributions are. #942

Job	Run time
build_lib_test (nightly)	51s
build_bin_spec (nightly)	1m 6s
build_wasm (nightly)	48s
clippy_format	2m 22s
build_wasm (beta)	55s
build_bin_spec (beta)	1m 6s
build_lib_test (beta)	1m 12s
build_wasm (stable)	50s
build_lib_test (stable)	1m 6s
build_bin_spec (stable)	1m 2s
lockfile	1m 7s
build_lib_test ($MSRV)	3m 21s
build_wasm ($MSRV)	2m 34s
build_bin_spec ($MSRV)	3m 8s
no_features_build_test (nightly)	34s
no_features_build_test (beta)	35s
no_features_build_test (stable)	23s
no_features_build_test ($MSRV)	2m 15s
	25m 15s

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