Adding charms' libraries as Git submodules, in cases where the development is being done in a Git repository

The current documentation on how to create a Juju-based execution environment with Proxy Charms explains the directories and files that must be created for a new Juju Proxy Charm. However, the commands for adding the Charmed Operator Framework and charms.osm libraries only take into account that they will not be used in a developer's own Git repository (because the displayed commands clone the libraries repositories).

Therefore, the small modification I have done was to duplicate the set of commands related to the directories and files creation, but now with two possibilities: one for cloning the libraries' repositories, and the other to add them as Git Submodules. This way, the developer can copy and past the preferable set of commands more adapted for their needs.