Update README.md

[anjanbharadwaj]

I spent a decent amount of time trying to understand why I wasn't able to convert my dataframes to a SCESet object, until I realized scater had switched from SCESet objects to SingleCellExperiment objects. Would help if you could change that on the README (since it isn't completely straightforward to convert between the two). I understand that this is an old project though, so that's probably why.