docs: link to development docs in README #891

Job	Run time
Clippy	1m 31s
Rustfmt	5s
Test (MSRV)	8m 29s
Build docs	2m 33s
Test (stable)	3m 37s
Test (beta)	3m 22s
Test (nightly)	4m 0s
Test (macOS)	4m 13s
Test (Windows)	8m 1s
Test (no features)	1m 38s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	2m 30s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	2m 3s
	42m 2s

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