Add a Free Software/open source license to the project

[cjuner]

Please add a Free Software/open source license to your project, so others may use the buildpack legally in countries with copyright laws. All existing contributors should agree on a license for others to be able to rely on it as being binding and intended.

• The Free Software Foundation has a guide to choose a license: https://www.gnu.org/licenses/license-recommendations.en.html
• GitHub has a website that can help you choose a license for your project: https://choosealicense.com/