docs: document how to land ICU floating patches

[srl295]

per: #26090 (review)

document how to land floating patches to ICU.

// @refack

[richardlau]

@srl295 I think the most logical place for this is https://github.com/nodejs/node/tree/master/doc/guides -- See the existing maintaining-npm.md and maintaining-V8.md for what's currently documented for npm and V8.

[srl295]

@richardlau yes, though there's already tools/icu/README.md. I think it makes sense to move some content from the README into a new guide.