docs: link to development docs in README (#214) #892

Job	Run time
Build docs	4m 27s
Test (MSRV)	7m 58s
Rustfmt	6s
Clippy	1m 5s
Test (stable)	3m 20s
Test (beta)	4m 7s
Test (nightly)	3m 25s
Test (macOS)	5m 26s
Test (Windows)	9m 25s
Test (no features)	2m 20s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	2m 21s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	2m 44s
	46m 44s

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