Hacktoberfest is a contest by DigitalOcean (VPS) where you win a t-shift if you make at least five pull requests on Github within October.
This is the fifth time that Hacktoberfest is running. Last year, they gave away 30.000 t-shirts.
Any pull request that you make on Github will count (see also full rules).
If you want to make a contribution to LXC/LXD, see the available projects at https://github.com/lxc
Even a documentation contribution would be fine for this.
Personally, I am interested in the distrobuilder repository (https://github.com/lxc/distrobuilder),
which is the new tool to create container images.
The tool works, but, for example, the README.md can be expanded to show how to use.