docs: add note about sponsors/support #1778

Job	Run time
Docs	1m 37s
Check Style	13s
MSRV	2m 9s
linux / nightly	1m 16s
Android	30s
linux / stable	55s
unstable features	21s
WASM	1m 41s
linux / beta	1m 18s
macOS / stable	1m 38s
windows / stable-x86_64-msvc	3m 46s
windows / stable-i686-msvc	3m 54s
windows / stable-x86_64-gnu	10m 59s
windows / stable-i686-gnu	9m 49s
feat.: default-tls disabled	47s
feat.: rustls-tls	59s
feat.: rustls-tls-manual-roots	1m 2s
feat.: native-tls	53s
feat.: default-tls and rustls-tls	49s
feat.: cookies	1m 12s
feat.: blocking	1m 11s
feat.: gzip	1m 5s
feat.: brotli	1m 2s
feat.: deflate	58s
feat.: json	1m 6s
feat.: multipart	55s
feat.: stream	1m 7s
feat.: socks/default-tls	50s
feat.: socks/rustls-tls	57s
feat.: hickory-dns	1m 3s
CI is green	1s
	56m 3s

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