Add usage guide

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This library can be added as a git submodule to any KiCad project that wants to use these symbols/footprints. When doing this the symbol library and footprint library paths need to be modified to point to the corresponding locations in this repository. Using project-level relative paths seems like the preferred approach since it allows to setup both the project and the libraries by simply cloning the project repo and running git submodule update --init --recursive.

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The electronics-prototyping repository can serve an example on how to configure KiCad projects with this library.