Talor Hornby from the Zcash development team has just released a very good quick-start guide for anyone who wants to work on squashing bugs in the Zcash protocol.

This guide is particularly nice in that it also highlights the key areas where Zcash has changed from the Bitcoin Core consensus rules and it shows which components of libsnark are being used.

You can find the new guide here:

 https://github.com/zcash/zcash/wiki/Security-Auditor-Quick-Start