Add a `CONTRIBUTING.md` file

Given that

> These types have been auto generated using an internal script within UXP codebase.

It seems like opening PRs, making changes manually doesn't make a lot of sense. I assume that the preferred way to contribute is therefore to open issues in this repo? A specification of this, whatever the preferred way for the community to contribute may be, in a `CONTRIBUTING.md` can forestall folks from the community investing time into doing work that cannot actually get merged.

Start by reading the issue body and reviewing how the repository describes its auto-generated UXP types. Define the contribution path maintainers want to support, especially whether changes should be proposed through issues rather than manual pull requests. Done means a CONTRIBUTING.md clearly records that guidance so contributors know how to proceed.

Written by the indexing model from the issue text.