Add weights in readme

[JJ Chen]

Since the help file is still missing, maybe it would be worthwhile to add in readme the newly supported weight option?

By the way, I really like the new name sumup; I can easily replace all by su or sum commands by typing a few more letters. Thank you again for the package!

[Matthieu Gomez]

Thanks. I have just created a help file and submitted the package to SSC.