Add fabric.mod.json spec from Fabric Wiki

[Juuxel]

Almost a direct copy converted from Dokuwiki format to Markdown with Pandoc. The only content change is that I removed the mention of "an array containing such [mod definition] objects". However, I'm not sure if this file can be directly copied here license-wise?

[Juuxel]

Looking at wiki edit history you seem to be the only editor of the page

@i509VCB That's because it's a moved page. The original page's history also shows asie and falseresync there.

[i509VCB]

I see that we will need to poke asie and ~~false~~ (done) about permission to use the spec from wiki.

I think we should get them to agree to license the spec document under CC0 (and probably the same with all the spec documents in loader for future) so we can have this spec send stuff back to the wiki? That is a question I would like to have discussed.

[falseresync]

As i5 asked, I hereby grant any and all permission to do anything and everything with this spec :>

[Juuxel]

@asiekierka Do you agree to licensing this spec under CC0?

[liach]

@sfPlayer1 Should we include these here or just a simple readme asking people to visit fabric wiki instead?