Add documentation for GLPK.Optimizer

Hi all,

There's currently no documentation for GLPK.Optimizer. All we have is a quick mention in the [JuMP Quickstart Docs](http://www.juliaopt.org/JuMP.jl/v0.19.0/quickstart/#Quick-Start-Guide-1).

Any advice on additional options for the optimizer would be appreciated!

Thanks,
Oliver