[docs] update roadmap for post v1.0 (#2883)

docs/src/developers/roadmap.md

@@ -5,6 +5,32 @@ community. The JuMP developers have compiled this roadmap document to
 share their plans and goals. Contributions to roadmap issues are especially
 invited.
 
-```@meta
-# TODO: the roadmap should be re-written for the post-JuMP 1.0 world.
-```
+Most of these issues will require changes to both JuMP and MathOptInterface, and
+are non-trivial in their implementation. They are in no particular order, but
+represent broad themes that we see as areas in which JuMP could be improved.
+
+ - Make nonlinear programming a first-class citizen. There have been many issues
+   and discussions about this: currently nonlinear constraints are handled
+   through a `MOI.NLPBlock` and have various limitations and restrictions.
+   - [https://github.com/jump-dev/JuMP.jl/issues/1185](https://github.com/jump-dev/JuMP.jl/issues/1185)
+   - [https://github.com/jump-dev/JuMP.jl/issues/1198](https://github.com/jump-dev/JuMP.jl/issues/1198)
+   - [https://github.com/jump-dev/JuMP.jl/issues/2788](https://github.com/jump-dev/JuMP.jl/issues/2788)
+   - [https://github.com/jump-dev/MathOptInterface.jl/issues/846](https://github.com/jump-dev/MathOptInterface.jl/issues/846)
+   - [https://github.com/jump-dev/MathOptInterface.jl/issues/1397](https://github.com/jump-dev/MathOptInterface.jl/issues/1397)
+ - Add support for coefficient types other than `Float64`:
+   [https://github.com/jump-dev/JuMP.jl/issues/2025](https://github.com/jump-dev/JuMP.jl/issues/2025)
+   Since the very beginning, JuMP has hard-coded the coefficient type as
+   `Float64`. This has made it impossible to support solvers which can use other
+   types such as `BigFloat` or `Rational{BigInt}`.
+ - Add support for constraint programming:
+   [https://github.com/jump-dev/JuMP.jl/issues/2227](https://github.com/jump-dev/JuMP.jl/issues/2227)
+   JuMP has a strong focus on linear, conic and nonlinear optimization problems.
+   We want to add better support for constraint programming.
+ - Add support for multiobjective problems:
+   [https://github.com/jump-dev/JuMP.jl/issues/2099](https://github.com/jump-dev/JuMP.jl/issues/2099)
+   JuMP is restricted to problems with scalar-valued objectives. We want to
+   extend this to vector-valued problems.
+ - Refactor the internal code of JuMP's macros. The code in `src/macros.jl` is
+   some of the oldest part of JuMP and is difficult to read, modify, and extend.
+   We should overhaul the internals of JuMP's macros---without making
+   user-visible breaking changes---to improve their long-term maintainability.