Add guidelines for naming packages

Ref: https://groups.google.com/forum/#!topic/julia-dev/Qw1eaMUKYas

Guidelines courtesy of @johnmyleswhite and @joehuchette
The reason for the name change from CVX to Convex was on request from the author of the CVX matlab package, not for increased clarity. I'd also claim that Convex.jl is one particular way to model convex optimization and isn't the "unique source of truth", so I'm not sure it's a good example.