Mention antigen-hs in readme

Since antigen was what made my zsh-loading time unbearable, I thought it would be nice to share this little plugin manager I wrote on my own. I would also like to invite other programmers to share their own plugin manager implementations here (as afaict, antigen is the only well-known software for this).
Merge pull request #84 from Tarrasch/patch-2

Mention antigen-hs in readme