Add a CONTRIBUTING.md file for package:dart_mcp

[jakemac53]

Should point at the schema files, and outline the general process for developing this package.

[devoncarew]

Consider naming it something other than CONTRIBUTING.md, since a file w/ that name is handled specially by github, and we often have repo std ones and a fallback one in http://github.com/dart-lang/.github.

[jakemac53]

Hmm, what should we call it? I think this is the standard file people are going to look for when trying to understand the development process for the repo?

What if it also links to some parent one?

[devoncarew]

I think a link would be fine; I just want to avoid having multiple out of date copies. https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md should be canonical; you could put content into something like DEVELOPING.md, SETUP.md, ..., or, reference https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md and have just have local setup instructions in your file.

(I'm a little less concerned w/ having a complete copy of https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md in a repo root; we have many fewer repos now, so updating them would be tractable).