Update contributing guidelines

With the workbench, the default branch now is the `main` branch instead of the `gh-pages` branch. We will have to update the existing [CONTRIBUTING.md](https://github.com/ImperialCollegeLondon/rse_introductory_git_course/blob/main/CONTRIBUTING.md) file to reflect this.

Also it will be nice to have a link to this repository's URL in the contributing guidelines.

