Additional PuTTY Connection Info

Added a step to generate a .ppk file using the PuttyGen program included with PuTTY and select the generated .ppk file in the Auth section of PuTTY. Also, added a note to enter 'vagrant' as a username if prompted when trying to SSH via PuTTY.

Ran through the lesson on both PC/Mac and that was the only roadblock I encountered. Keep up the good work.