Move different licensing content from README to LICENSE file

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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 moves the reference to Creative Commons licensing from
the LICENSE file to the README, which allows Licensee to successfully
identify the license type of GitLab CE as MIT.

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

As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/42891

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 updates the LICENSE file so that it contains the exact text
of the MIT license. It also moves the reference to third-party software
licensing to the README.

These changes allow Licensee to successfully identify the license
type of GitLab's codebase as MIT.

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