Add link to `AgentBuilder` in README #1861

Job	Run time
build_versions (stable)	34s
build_versions (beta)	34s
cargo-deny (aarch64-apple-ios)	23s
build_versions (1.61.0)	17s
Test	44s
cargo-deny (aarch64-linux-android)	21s
Docs	26s
cargo-deny (i686-pc-windows-gnu)	25s
Test (json)	43s
cargo-deny (i686-pc-windows-msvc)	19s
Test (charset)	48s
Lint	20s
cargo-deny (i686-unknown-linux-gnu)	19s
Test (cookies)	50s
cargo-deny (wasm32-unknown-unknown)	20s
Test (socks-proxy)	43s
cargo-deny (x86_64-apple-darwin)	18s
Test (native-certs)	44s
cargo-deny (x86_64-apple-ios)	18s
Test (gzip)	45s
cargo-deny (x86_64-pc-windows-gnu)	22s
Test (brotli)	46s
cargo-deny (x86_64-pc-windows-msvc)	18s
cargo-deny (x86_64-unknown-linux-gnu)	23s
cargo-deny (x86_64-unknown-redox)	17s
Test (http-crate)	46s
Test (http-interop)	46s
Test (http-crate http-interop)	45s
Test (tls)	49s
Test (tls, json)	49s
Test (tls, charset)	1m 10s
Test (tls, cookies)	53s
Test (tls, socks-proxy)	50s
Test (tls, native-certs)	46s
Test (tls, gzip)	48s
Test (tls, brotli)	50s
Test (tls, http-crate)	48s
Test (tls, http-interop)	52s
Test (tls, http-crate http-interop)	55s
Test (native-tls)	45s
Test (native-tls, json)	46s
Test (native-tls, charset)	52s
Test (native-tls, cookies)	56s
Test (native-tls, socks-proxy)	48s
Test (native-tls, native-certs)	47s
Test (native-tls, gzip)	47s
Test (native-tls, brotli)	49s
Test (native-tls, http-crate)	49s
Test (native-tls, http-interop)	48s
Test (native-tls, http-crate http-interop)	50s
	33m 1s

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