Propose MPLv2 Open Source License

[NotAlexNoyle]

While auditing my modpack for open source compliance, I noticed that AdvancedChatHUD is missing a license. Without an open source license, your code is only source-available, and the copyright is assumed to be owned by you. I propose setting your project free by adopting the FOSS MPLv2 License, the same as the parent project AdvancedChatCore. Thanks for your consideration.

[Purrrpley]

I think it's already technically licensed under the MPLv2 already, it's just missing a LICENSE.txt file:

• There's a HEADER.txt file, which references the MPLv2. It seems to be some auto-generated copyright header thing, which means ...
• ... each (or at least the ones I checked) source code file is already explicitly licensed under the MPLv2 because they have that copyright header at the top of them.
• On Modrinth it's marked as being MPL-2.0: https://modrinth.com/mod/advancedchathud

If I'm correct this PR isn't even actually suggesting to re-licensing the project or anything, it's just making what it already is clearer