Add a section for Godep in CONTRIBUTING.md

What does this MR do?

Documents a (very conservative) process for keeping Godeps up to date when submitting MRs

Why was this MR needed?

MRs often come with changes to Godeps. Sometimes the wrong version of Go was used to make the change, sometimes no-longer-used dependencies in vendor/ are not cleaned out. This procedure recommends re-generating Godeps from scratch each time an MR touches dependencies, so contributions keep Godep as up to date as possible.

Are there points in the code the reviewer needs to double check?

Is there a better process? This is quite bandwidth-intensive.

/cc @tmaczukin