Add license to mmx

[PhoebeHui]

Suggest to add a license to mmx, some license identifiers https://spdx.org/licenses/.

[soruevans]

Hmm it appears the licenses are stated in the Readme and the header files themselves

The only issue is that json.h says public domain, but the Readme says that json.h is under the zlib license.

[PhoebeHui]

@eater-of-souls, thanks for the information! so for json.h, does it apply zlib license?

[soruevans]

No problem!

I think we’ll have to wait for vurtun to clarify for json.h

[soruevans]

After closer inspection it looks like json.h used to be under zlib and then changed to public domain. The Readme just needs to be updated.

This commit came first:

72dadca

Followed by this commit:

e876e64

[PhoebeHui]

Thanks for your help!