Document procedure to add library to a wgt when built with SDK (just bare minimum procedure)

Added a piece of documentation specifying where to put external libraries to be loaded by the binder with a binding.

To include an external library to a widget. Then, in your build procedure just before calling wgtpkg-pack utility, you have to copy your external library compiled for the target, in the "lib" directory just under the root package directory.

Documentation has to be updated WIP