Add license (#9)

Reviewed-on: https://gitea.com/gitea/runner-images/pulls/9

CONTRIBUTING.md

@@ -0,0 +1,28 @@
+# Contribution Guidelines
+
+<details><summary>Table of Contents</summary>
+
+- [Contribution Guidelines](#contribution-guidelines)
+  - [Introduction](#introduction)
+  - [Developer Certificate of Origin (DCO)](#developer-certificate-of-origin-dco)
+
+</details>
+
+## Introduction
+
+This project is the official Gitea Actions runner images. This document explains how to contribute changes to the project. \
+Sensitive security-related issues should be reported to [security@gitea.io](mailto:security@gitea.io).
+
+## Developer Certificate of Origin (DCO)
+
+We consider the act of contributing to the code by submitting a Pull Request as the "Sign off" or agreement to the certifications and terms of the [DCO](DCO) and [MIT license](LICENSE). \
+No further action is required. \
+You can also decide to sign off your commits by adding the following line at the end of your commit messages:
+
+```
+Signed-off-by: Joe Smith <joe.smith@email.com>
+```
+
+If you set the `user.name` and `user.email` Git config options, you can add the line to the end of your commits automatically with `git commit -s`.
+
+We assume in good faith that the information you provide is legally binding.