Add SpineOpt.jl (#860)

A Julia package containing a state-of-the-art energy system optimization model. https://github.com/spine-tools/SpineOpt.jl

README.md

@@ -402,6 +402,7 @@ energy system designs and analysis of interactions between technologies.
 - [Energy Transition Engine](https://github.com/quintel/etengine) - Calculation engine for the Energy Transition Model.
 - [enerflow](https://github.com/rebase-energy/enerflow) - An open-source Python framework that enables energy data scientists and modellers to write modular and reproducible energy models that solves sequential decision problems.
 - [Energy System Simulation and Optimization](https://github.com/Akkudoktor-EOS/EOS) - An open-source energy optimization system that integrates renewable energy sources, such as photovoltaic (PV) systems, battery storage, and smart load management solutions.
+- [SpineOpt.jl](https://github.com/spine-tools/SpineOpt.jl) - A highly adaptable modelling framework for multi-energy systems written in Julia.
 
 ### Energy Markets
 - [Grid Singularity Energy Exchange](https://github.com/gridsingularity/gsy-e) - An interface to download and deploy interconnected, grid-aware energy marketplaces.