docs: document lack of sdist support

[mwgamble]

Using this plugin doesn't result in the pyproject.toml source tarball being updated. I guess this can be worked around by not publishing the source tarball? Most people will only want and need wheels anyway. It would be good to document this as a limitation though (I'm not sure if updating the pyproject.toml file is even feasible).

[danielknell]

i mean it is called stickywheel 😂 but when i sort some proper docs ill make sure to note that it wont work with sdists

[mwgamble]

Yeah, that did occur to me after I submitted the issue 😅 I still think it's worth mentioning though.