Added first sponsor logo (puppet labs)

Puppet Labs has agreed to be a gold sponsor, so I added the logo in the images directory and referenced it from the sponsor section on the website under the gold section. 

Kris and Toshaan are working on the payment transfer now. I also made a few tweaks around adding the logo in the sponsor section: added a comment with size instructions to keep logos lined up and added a little more text to make a better transition between sponsor logos and request to get new sponsors.