Mention ruby-build in README
Mention ruby-build in README

The command "rbenv install 2.1.5" requires the ruby-build plugin.  Add a
link to that.

dondonz/planner,lislis/planner,matyikriszta/planner,webycel/planner,spike01/planner,despo/planner,margOnline/planner,boddhisattva/planner,boddhisattva/planner,boddhisattva/planner,ndchorley/planner,despo/planner,spike01/planner,matyikriszta/planner,despo/planner,matyikriszta/planner,ndchorley/planner,spike01/planner,deniseyu/planner,codebar/planner,laszpio/planner,webycel/planner,laszpio/planner,dondonz/planner,dondonz/planner,despo/planner,laszpio/planner,deniseyu/planner,lislis/planner,codebar/planner,spike01/planner,webycel/planner,laszpio/planner,codebar/planner,lislis/planner,margOnline/planner,ndchorley/planner,codebar/planner,deniseyu/planner,margOnline/planner