Add license (#9)

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

CONTRIBUTING.md

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+# 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.

DCO

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+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.

LICENSE

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+Copyright (c) 2024 The Gitea Authors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.