These are not official, stable releases, these are automated builds from the master Git branch for testing. Builds are performed on macOS 10.12.6, Xcode 9.2, using the build script provided in the project repository.
There is a download-and-install script available to get and install the tools in one step. Run the following command in a terminal session (once you've audited the script yourself):
curl https://munkibuilds.org/latest3.sh | sh
If you use AutoPkg, there are also recipes to get the latest builds.
Munki is licensed under the Apache 2.0 license. Visit the Munki project site on GitHub.
Maintained by Tim Sutton.