[FR] explain how to update external libraries that are not managed by Library Manager Registry

[DerAndere1]

I created a plain C project for the Atmel AVR platform that depended on libraries which are in git repositories but not in the PlatformIO Library Manager Registry. I added these dependencies using the libdeps entry in platformio.ini .

I could not find the following information in the documentation:

- If a library is available from a version controlled repository, AND
- if the dependency is not available in the PlatformIO Library Manager Registry, AND
- if the depency was specified using the libdeps entry in platformio.ini, 

such library can be updated by deleting the subfolder [your_project_folder]/.pio/libdeps/[your_env]/[your_library_folder]  
of your project. The updated library should be added back automatically after some seconds. 

A good place for that information would be here:
https://github.com/platformio/platformio-docs/blob/develop/librarymanager/quickstart.rst#project-dependencies

and/or here:
https://github.com/platformio/platformio-docs/blob/develop/projectconf/section_env_library.rst