add copying to list of license file names

[axon-consultancy]

Some project use a COPYING file for their license. This PR adds this file to the license list that is used to find files to scan. As a reference, my project has 72 dependencies for which the detector was able to detect 64. With the copying file it detected licenses in all repo's, so it looks like it's quite common.

The COPYING file seems to be a convention used in the GNU project (http://www.gnu.org/licenses/gpl-howto.html).

example: https://github.com/aristanetworks/goarista

[vmarkovtsev]

Thanks a lot for this contribution! I am back from coronavirus disaster, sorry for making you wait.

Can you please sign the DCO: https://github.com/src-d/go-license-detector/pull/74/checks?check_run_id=588276834

[taleinat]

A few lines above in the same list, there is already "copy(left|right|ing)", which should match "copying". If that's broken, it should be fixed. In any case, adding "copying" to this list doesn't seem appropriate...

[mcuadros]

@axon-consultancy will be great if you can submit your PR to https://github.com/go-enry/go-license-detector, we moved the project.