docs: link to development docs in README #890

Job	Run time
Build docs	2m 27s
Rustfmt	10s
Clippy	1m 37s
Test (MSRV)	7m 55s
Test (stable)	3m 16s
Test (beta)	4m 54s
Test (nightly)	4m 32s
Test (macOS)	4m 59s
Test (Windows)	7m 38s
Test (no features)	2m 40s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 55s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 49s
	43m 52s

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