Reference to non-existing LICENSING file in README

When I use the dev plugin to initialize a new plugin, the generated README file ends with the line

`See the LICENSING file for details`

However, the license file that is generated is named LICENSE, not LICENSING. So it is necessary to either change the text at the end of skel/README, or rename the file skel/LICENSE, or both.

Then the question is: Should the file be named LICENSE, or LICENSING, or something else?

In the "GPL Howto" (https://www.gnu.org/licenses/gpl-howto.html), it is recommended to put the license text in a file named COPYING. My suggestion would be to follow GNU's convention and use COPYING as the file name. This is also consistent with DokuWiki itself, which has the GPLv2 license text in a file named COPYING.

(Note that while my comments here apply to the files skel/LICENSE and skel/README used for initializing a new plugin, the exact same mistake also appears in the dev plugin's own README file, and should be fixed there as well.)