[docs] update roadmap for post v1.0

[odow]

Preliminary sketch. We should discuss in a monthly developer call.

[codecov]

Codecov Report

Merging #2883 (206c799) into master (9b4847a) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2883      +/-   ##
==========================================
- Coverage   95.23%   95.16%   -0.07%     
==========================================
  Files          43       43              
  Lines        5692     5755      +63     
==========================================
+ Hits         5421     5477      +56     
- Misses        271      278       +7     

Impacted Files	Coverage Δ
src/quad_expr.jl	96.00% <0.00%> (-2.09%)	⬇️
src/aff_expr.jl	94.71% <0.00%> (-1.91%)	⬇️
src/variables.jl	98.70% <0.00%> (+0.01%)	⬆️
src/print.jl	98.53% <0.00%> (+0.01%)	⬆️
src/JuMP.jl	95.61% <0.00%> (+0.03%)	⬆️
src/operators.jl	96.13% <0.00%> (+0.61%)	⬆️
src/mutable_arithmetics.jl	88.57% <0.00%> (+0.69%)	⬆️
src/optimizer_interface.jl	82.00% <0.00%> (+0.75%)	⬆️
src/solution_summary.jl	98.57% <0.00%> (+3.04%)	⬆️

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[odow]

@pulsipher suggests: refactoring the macro code