Add the full text of the license (GPL v2).

[LinoMastro]

Note: this is not a license change, the FreeDOS kernel is already released under the GPL v2.

The license text comes unmodified from https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt (LF converted to CRLF).

[PerditionC]

The full license is in docs as copying. I'm not opposed to moving the license file to make it more clear, but would like to hear from some other kernel devs opinions on it.

[LinoMastro]

Oops, sorry I missed the copy of the license in docs/.

It could still be useful to move it to the top-level directory and rename it to LICENSE to make it easier for automated tools to find it. E.g. github doesn't find it in the current location but it does notice it after this commit; it looks like this:

Let me know what you think, I don't have a strong opinion either way.

[LinoMastro]

FYI, I've updated this PR to simply move the existing "copying" file with the text of the GPLv2 to the top-level directory, to make it autodetectable by github and other automated tools, and to follow the same style as https://github.com/FDOS/diskcopy and other FreeDOS repositories.