Add license identifier to readme

[MyreMylar]

fixes #1737

I know @Starbuck5 wants to rewrite the readme anyway, but I thought I'd toss this into the pot as a thing to clarify.

The debate here would be between the two variants:

• LGPL-2.1-only
• LGPL-2.1-or-later

I have no strong feelings on this topic but I can see why some users might want it clarified for legal reasons.

[andrewhong04]

LGTM

[MyreMylar]

I should note that @ankith26 expressed a preference on discord for LGPL-2.1-only on the basis that it is the effective status quo.

As I said before I don't really mind either way this was originally raised by @ruben2020 so perhaps they have stronger feelings on the topic.

[ruben2020]

@MyreMylar If you're fine either way, I would recommend LGPL-2.1-or-later as it is more flexible for users than LGPL-2.1-only.

[ankith26]

We do have a restriction of not changing the terms of the license, and I don't want this PR to be interpreted as so.

Hence I believe we are more or less forced to maintain status quo, and I believe LGPL-2.1-only achieves it, even though technically both LGPL-2.1-only and LGPL-2.1-or-later have identical license text (which is what creates this confusion in the first place).

I would like to hear @Starbuck5 s opinions here to settle the matter

[Starbuck5]

I'm fine to go 2.1-or-later.

Thoughts:

• My theory was that Pete Shinners chose this license because it's the same one as SDL1 used. Looking at SDL1, they almost always refer to it as the "LPGL", not "LPGL v2", so the spirit of the "or later" is there.
• This is a matter up to interpretation, so it's fine for us to interpret it this way
• This is somewhat academic, will not make a difference to many people (if it makes a difference to anyone)