updated options section in readme

prefix option is messing inside readme. Also maybe is a little bit confusing to have ``key`` and ``prefix`` options. Maybe would be better to have one option for both client and server store key. Yes, someone can say...wait but with two keys we have more flexibility....hm, I am not sure how many people will use this, but I am pretty sure that there will be more people which will just expect single key for both client and server store.
Merge pull request #14 from ivpusic/patch-1

updated options section in readme
update readme
Merge branch 'master' of github.com:koajs/koa-session