Update CONTRIBUTING.md

Corrected the location of the upstream branch.
Merge branch '3.0-pinned-containers' of https://github.com/angular-ui/ng-grid into 3.0-pinned-containers
Bugfix: .html() only works with jquery present; changing to append()
Merge branch '3.0-pinned-containers' of https://github.com/angular-ui/ng-grid into 3.0-pinned-containers