[Development] Disabling Pull Requests on GitHub Mirrors

[Igor Khanin]

Hello,

I'm not sure who is responsible for administering the mirrors of Qt 
source code in GitHub under https://github.com/qt, but now that GitHub 
has made this an option - I think that the pull requests tab should 
probably be disabled in the repositories there. People still seem to 
routinely try to make a contribution through the usual GitHub flow and 
then wonder why their PRs are ignored. This could be coupled with 
copying the CONTRIBUTING.md file from the top level qt5 repository also 
to the main module repositories (qtbase,  qtdeclarative, etc) as GitHub 
highlights its' content in the UI.

All the best,
Igor