Added LGPLv3 to top-level of repo.

This is a possibly controversial MR. Feel free to reject.

At the moment, we say that 'you should have received a version of the LGPLv3 with this software'. We do not currently distribute a copy of the LGPL. We distribute a copy of the GPLv3 with our COPYING text file.

I propose that we either:

• Move COPYING to LICENSE.md and include a markdown version of the GPLv3, or
• Remove COPYING and leave this version of the LGPLv3.

@pdraper @matthieu @nnrw56 please could you comment on this.