I suggest to restructure the repo a little bit. To have a clearer structure, there should be a new folder "src" or "content" that contains the content files. The new structure would be like so:
...
img/
|- ...
tools/
|- ...
src
|- toc.md
|- Introduction.md
|- Contributions-to-existing-projects.md
|- starting-oss-projects.md
|- references.md
LICENSE
README.md
CONTRIBUTING.md
...
With that it's easier (also for new contributors) to figure out where the relevant content files are located.
What do you think about that?