Clarify licence: LGPL-2.1-only

[Olf0]

Addresses second point of issue #50.

[Olf0]

Oh well, that was intentionally marked as a draft PR:

• It was not at all complete.
• I think I wanted to rectify some change I did.

I will revisit this again in a couple of weeks and pose another PR, which finalises this task #50.