Documentation.md still says Trimble accepts PRs to the stubs repo

[DanRathbun]

Stubs Documentation Issue

Documentation.md file still say Trimble accepts PRs to the stubs repo, viz:

We accept improvements to the API documentations via Pull Requests to the https://github.com/SketchUp/ruby-api-stubs repository.

Upon a Pull Request we will review the changes and maybe request some tweaks. Once we're ready to merge this we will pull your development branch and run a tool which will back-port the documentation changes to our C++ source.

Then we'll regenerate the documentation and stubs - finally closing the Pull Request.