Add license file

[tbertels]

This was requested on #7 and it would also allow reusing content on Wikipedia.

[TheGiraffe3]

I believe GitHub normally has a file name of LICENSE.
But it does work like this, so that's probably fine.

[mOctave]

ES seems to use license.txt too...
Yay ES for not following any standards, like usual.

[warp-core]

I asked MZ about adding a license here by email and he said he gave his permission for GPL3 or a Creative Commons license (CC-BY-SA-4.0, for example) suggesting that the latter may be more appropriate given that a lot of the stuff around here is content, not code.

We might want to do something like what endless-sky itself does, and use GPL for the code and CC-BY-SA-4 for the non-code stuff (things like screenshots or sprite images are CC-BY-SA-4 already).

MZ's permission covers the person who I think is responsible for most of the stuff here, but there are some others we'll need permission from. Off the top of my head, @Amazinite and @Saugia have written recent release blog posts, and @beccabunny has also written a blog post. There are likely others we'll need to ask, as well.

[tbertels]

I've switched to CC-BY-SA-4.0 per warp-core's comment.
CC-BY-SA-4.0 content can be converted to GPLv3 but not the other way
around.
https://creativecommons.org/2015/10/08/cc-by-sa-4-0-now-one-way-compatible-with-gplv3/

[TheGiraffe3]

CC-BY-SA-4.0 is the established license for Endless Sky, so I'd say it fits pretty well.

[Amazinite]

Per the contributors list, this is who we'd need to get approval from.

Contributor	Approval Received?
@warp-core	Yes
@tehhowch
@MCOfficer
@quyykk
@tibetiroka
@TheGiraffe3	Yes
@petervdmeer	Yes
@HeadPanda
@ZufengW	Yes
@CodeDraken	Yes
@shnwnd
@flaviojs
@lumbar527
@bene-dictator	Yes
@Amazinite	Yes
@beccabunny
Michael Zahniser	Yes

[tibetiroka]

Is there any reason to not allow both GPLv3+ and CC-BY-SA 4.0 (+?)

[CodeDraken]

Per the contributors list, this is who we'd need to get approval from.

Contributor Approval Received?
@CodeDraken ✔️

I added nothing, but you have my approval anyway 😂

[ZufengW]

You have my approval too.

However, that contributor list includes some very small and/or old contributions (such as mine). Maybe you don't need approval from all of them.

[petervdmeer]

You have my approval too.

[bene-dictator]

Approval from me as well