Add high level design information (#757)

conception/README.md

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+# Xpansion design informations
+
+## Domains
+
+Xpansion aims at performing investment simulations for Antares studies. This means that the core domain of xpansion
+could
+be described as : **investment simulations for Antares studies**
+This core domain is composed of subdomains:
+
+- Investment strategy: how to describe the investment strategy based on an Antares Simulation
+- Linear optimization problem-solving
+
+## Mapping design with domains
+
+| Domain                                     | Component           | Details                                                                                                                                                                     |
+|--------------------------------------------|---------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| Investment simulations for Antares studies | Xpansion as a whole |                                                                                                                                                                             |
+| Investment strategy                        | Problem Generation  | Investment strategy is defined by candidates. Those candidates are used to enrich an Antares study simulation. It produces a linear optimization problem                    |
+| Linear optimization problem-solv ing       | Benders             | Benders solve optimization problems. It is agnostic of the "business" of the core domain. Mathematical outputs need to be translated as business values in other components |
+
+## Conception folder
+
+In the conception folder you will find design documentation.
+
+### Architecture decision records
+
+Also called ADR. A list of design decisions with date, status and motivation. This allows going back to ADR to
+understand
+why choices have been made. Choices are not irrevocable, they can be deprecated in favor of new choices. The goal of
+ADRs
+is exactly to follow the evolution of these choices.
+
+### C4
+
+Various C4 diagrams following [C4 model conventions](https://c4model.com/)
+
+### SequenceDiagrams
+
+Various sequence diagrams to help understand the flow of the application
+
+### SimulationOutputs
+
+Various example of simulation outputs
+