Add docs badge (#307)

Author: ericphanson

README.md

@@ -2,6 +2,8 @@
 
 [![Build Status](https://travis-ci.org/JuliaOpt/Convex.jl.svg?branch=master)](https://travis-ci.org/JuliaOpt/Convex.jl)
 [![Coverage Status](https://coveralls.io/repos/JuliaOpt/Convex.jl/badge.svg?branch=master)](https://coveralls.io/r/JuliaOpt/Convex.jl?branch=master)
+[![](https://img.shields.io/badge/docs-stable-blue.svg)](https://www.juliaopt.org/Convex.jl/stable)
+[![](https://img.shields.io/badge/docs-dev-blue.svg)](https://www.juliaopt.org/Convex.jl/dev)
 
 **Convex.jl** is a [Julia](http://julialang.org) package for [Disciplined Convex Programming](http://dcp.stanford.edu/). Convex.jl can solve linear programs, mixed-integer linear programs, and DCP-compliant convex programs using a variety of solvers, including [Mosek](https://github.com/JuliaOpt/Mosek.jl), [Gurobi](https://github.com/JuliaOpt/Gurobi.jl), [ECOS](https://github.com/JuliaOpt/ECOS.jl), [SCS](https://github.com/JuliaOpt/SCS.jl), and  [GLPK](https://github.com/JuliaOpt/GLPK.jl), through the [MathProgBase](http://mathprogbasejl.readthedocs.org/en/latest/) interface. It also supports optimization with complex variables and coefficients.