Added more details on how to work with master

It would just be nice for anybody else who might bump into this problem to get some help.  Some of the comments are applicable in all cases (i.e. run ember init is just a good idea right) and the other one (deleting vendor) can't hurt either.  So why not help the next poor sap out who bumps into this.  Once you have a nice upgrade document this could be cleaned up... just a suggestion.

But it makes me think:  what if ember init script checks for the existence of a vendor directory and puts up a warning.  That would solve everything nicely.