If you want to hack Sputnik using the latest version from source control, you are in the right place. If you simply want to install and use it, please follow the instructions on Installation.
Sputnik's repository is hosted on Github, at http://github.com/yuri/sputnik/. You can clone it (git clone git@github.com:yuri/sputnik.git), edit the code, check it into your local branch, then push it into your own clone of Sputnik on github. Then let us know when you've got something interesting.
Installing from Source
Installing from source works similar to the regular Installation, except that you will need to also pull code from git and symlink it into your Sputnik installation.
So, first install Sputnik per Installation instructions. Then do this (from the same directory where you installed it):
git clone git://github.com/yuri/sputnik.git mainline
mv share/lua/5.1/sputnik share/lua/5.1/sputnik.bak
ln -s mainline/sputnik/lua/sputnik share/lua/5.1/sputnik
You will now have a standard Sputnik installation, with one extra directory ("mainline"), which is the git repository. Files from this directory will have been linked into "rocks" directory. If you want to edit files, edit files in "mainline". (If you add new files, make sure to run ./bin/luarocks-admin make-manifest after that.) See standard installation for configuration instructions.
Submitting Patches
If you only want to submit a single patch to Sputnik, then the easiest thing to do is to generate the patch with git and email it to the Mailing List.
To do so, edit some of the files in mainline.git until you achieve the desired change. Then do this:
cd mainline
git diff -p > my_fix.patch
Then send us the content of my_fix.patch.
If you are planning on making lots of complicated changes, the best thing to do go to our project on gitorious and create a "clone" and email us the link to it when you are done. We can then just pull your changes from gitorious.