MRG: update plugin documentation for users #5502

Job	Run time
Check	31s
coverage	10m 56s
Lints (beta)	34s
Lints (stable)	36s
test (beta)	1m 39s
test (stable)	1m 37s
test (windows)	3m 36s
test (macos)	1m 26s
test_all_features	13m 35s
Run tests under wasm32-wasi	1m 23s
Check if wasm-pack builds a valid package for the sourmash crate	1m 24s
Check if cbindgen runs cleanly for generating the C headers	8m 23s
minimum_rust_version	8m 8s
Publish (dry-run)	6m 0s
	59m 48s

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