GitHub uses a library called Licensee to identify a project's license
type. It shows this information in the status bar and via the API if it
can unambiguously identify the license.

This commit modifies a few of Portainer's docs so that Licensee is able
to recognize the repository's license type. It updates LICENSE so that
it contains only the text of the zlib license. It also moves the info
concerning 3rd-party software to a new "Licensing" section in the
README.

Collectively, these changes allow Licensee to successfully identify the
license type of Portainer as zlib.

Signed-off-by: Andrea Kao <eirinikos@gmail.com>