add contributing doc

CONTRIBUTING.md

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+# Contribution Guidelines
+
+## Table of Contents
+
+- [Contribution Guidelines](#contribution-guidelines)
+  - [Introduction](#introduction)
+  - [Bug reports](#bug-reports)
+  - [Submit your design](#submit-your-design)
+
+## Introduction
+
+This document explains how to contribute changes to the this github project.  It assumes you have followed the [installation instructions](https://github.com/jwobith/docker-gitea).
+
+## Bug reports
+
+Please search the issues on the issue tracker with a variety of keywords to ensure your bug is not already reported.
+
+If unique, [open an issue](https://github.com/jwobith/docker-gitea/issues/new) and answer the questions so we can understand and reproduce the problematic behavior.
+
+## Submit your design
+
+Submissions and updates are welcome. If you want to change or add something, fork the repo or clone it to your own local machine. After making your edits, submit a [pull request](https://github.com/jwobith/docker-gitea) with a descriptive name and include some details of the changes made. 
+
+If you want to make a proposal or start a discussion, please [file an issue](https://github.com/jwobith/docker-gitea/issues/new). File an issue with your proposed change documented, and make sure to note in the title of the issue that it is a proposal.