You can now add open source licenses to your GitHub projects really easily.
GitHub have created choosealicense.com which explains the different open source licenses and why you might choose one over the other. You can now add a new file to your project straight from the repo page and if you type "LICENSE" into the file name box you will see the drop down shown below appear on the right-hand side. This copies the content of the chosen license into your file and you can click commit at the bottom to persist the change. Nice!