Add license

[Roshanjossey]

License will help with avoiding legal problems while collaborating on the development here.

We could use the same Mozilla Public License 2.0 as https://github.com/helix-editor/helix

[the-mikedavis]

Using MPL seems like too much for a website. I think it's more common to use something permissive like a Creative Commons license or a 0BSD. What do you think @archseer?