Adding pentesterlab as we do a lot of code review content

[snyff]

PentesterLab has a lot of content on code review.

This changes add one of the free exercises on code review and also a link to pentesterlab for the paid training.

I also do paid training online and in-person (https://pentesterlab.gumroad.com/) but I wasn't sure if I could add it (so I didn't).