Apply license from CurseForge Page

[Sukasa]

The README.md file says the repo has an 'open source license', but this isn't actually attached to the repo. This PR just adds the license from CurseForge.

I also just need this to have all my ducks in a row with modpack work and Modrinth's licensing requirements.

[marcel0ll]

It might not be on the right spot, but the licence is there: https://github.com/marcel0ll/CustomItems/blob/master/src/main/resources/LICENSE.txt