Adding Yahoo to README
Adding Yahoo to README

yui-knk/omniauth,poznyakandrey/omniauth,nbrustein/omniauth,omniauth/omniauth,toyokazu/omniauth_shibboleth,thomasmillergb/Year2-ruby-software-hut,sideci-sample/sideci-sample-omniauth,dougal/omniauth,likeberi/omniauth,ngpestelos/omniauth,github/omniauth,twalpole/omniauth,intridea/omniauth,fxposter/omniauth,mihar/omniauth-timekiwi,ayunav/omniauth,thomasmillergb/Year2-ruby-software-hut