Update README.md links (#231) #951

Job	Run time
Rustfmt	8s
Clippy	43s
Build docs	2m 27s
Test (MSRV)	1m 20s
Test (stable)	1m 58s
Test (beta)	1m 59s
Test (nightly)	2m 2s
Test (macOS)	5m 0s
Test (Windows)	5m 26s
Test (no features)	1m 11s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 17s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 18s
	24m 49s

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