Added how to use `+(KSPromise *)when:(NSArray *)promises` to readme
Added how to use `+(KSPromise *)when:(NSArray *)promises` to readme

kseebaldt/deferred,briancroom/deferred,Trueey/deferred,mluedke2/deferred,briancroom/deferred,briancroom/deferred,kseebaldt/deferred,mluedke2/deferred,Trueey/deferred,kseebaldt/deferred,mluedke2/deferred,Trueey/deferred