It's perfectly fine to add new feature to this project.
About the pull request, the idea is that the code is on a Git repository and that there is a history of changes and that you can propose new changes. To do that, you need a (free) GitHub account and make a fork of the repository bgrabitmap/demo. You will then have a copy on your account, where you can make changes.
To do the changes on a repository, it is easier to have a graphical user interface like GitKraken. This will make it easy for you to retrieve your forked repository, and when you make changes to save them (commit and push). Then you can make a pull request and someone else can review your changes, as have two pairs of eyes is always better, and then merge into the source repository.
Note that we may simplify this process a bit. For this demo repository, we could give you access to the Git repository, so you would not need to fork it. If you're new to Git, this could make it easier for you to get the hands on it. You would still need a (free) GitHub account, but there would not be a pull request, so you would just need to check that your code is robust before pushing it to the repository.