Add chapter links in the projects to hints

[cansavvy]

It would be great to have links in the projects that send students to the places in the chapter that cover what they need to know for the project.

This was a recommendation by a student.

[cansavvy]

1. Follow the instructions as if you were a student: https://datatrail-jhu.github.io/DataTrail/getting-data-project.html
2. Start with the countries_project.Rmd in the 03-cleaning_data folder, follow the instructions and do the project.
3. Take note of what is tricky or needs more guidance -- add the guidance if you know it. Add links to relevant chapters in the course material that might be hints for students.

Take good notes. Then we'll add those hints back to a pull request on here: https://github.com/datatrail-jhu/DataTrail_Projects_Solutions