GitLab

Move the copyright license file from the older 'COPYING' to the now
somewhat more common 'LICENSE.md', and reformat it from plaintext to
Markdown.  The license itself (GPL-2.0) remains unchanged.

I did this after going looking for the license and not seeing it, not
because it wasn't there but because I'm so used to looking for a file
named "LICENSE" or "LICENSE.md" these days that I totally missed it
under the name "COPYING".  And I grew up using the former!  So if
nowadays I miss it on the first pass, other people probably would too.