Support for building and running external apps (#112)

Documentation/external-apps.md

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+# External Apps
+
+## Building
+
+Steps to build:
+
+- Build the main project with `idf.py build` in the root folder of the project
+- Open a terminal at `ExternalApps/${AppName}`
+- The first time, run `./build.sh` (during consecutive runs, you can run `idf.py build`)
+- Find the executable `build/${AppName}.app.elf` and copy it to an SD card.
+
+## Developing apps
+
+Currently, only C is supported. The `TactilityC` project converts Tactility's C++ code into C.
+
+Methods need to be manually exposed to the ELF apps, by editing `TactilityC/Source/TactilityC.cpp`
+
+You can put apps in `Data/config` or `Data/assets` during development, because putting it on an SD card takes more effort.