Add README for the build system module

nimble_build_system/README.md

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+The nimble build system is a python module containing custom functionality for building the nimble hardware.
+
+This is divided into three sub-packages:
+
+* `cad` - This containts any helper functions for building the Nimble CAD models. This does not contain the scripts for generating specific CAD models and assemblies. These are held in the `mechanical` directory of the repository.
+* `orchestration` - This contains code for the Nimble project's interaction with [CadOrchestrator](https://gitlab.com/gitbuilding/cadorchestrator)
+* `utils` - This contains scripts assorted utility scripts, for example for updating the json configuration files.