Please clarify license explicitly: LGPL-3-only or LGPL-3-or-later?

[Growl1234]

AFAIK the license text itself is not clear on if it's *GPL-*-or-later or *GPL-*-only. Generally adding SPDX headers on every source file owned by ABACUS is the best solution, but a quick patch is to clarify in the readme.

[mohanchen]

OK, we will discuss this later.

[mohanchen]

We will talk about this in discussion section.