Add note about sdsnewlen(0, ...) to readme

[miasujii]

It's convenient that sdsnewlen accepts 0 as a first argument, giving us a way to create an empty buffer of a given length. It would be nice to document this in the readme.

I don't know how the wording should be, but it felt lazy to not make a PR for such a small change. Feel free to close this PR and use whatever wording you like ;-)