Add openZIM project CONTRIBUTING file

CONTRIBUTING.md

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+# Contributing Guidelines
+
+We love improvements to our tools! There are a few key ways you can help us improve our projects:
+
+### Submitting Feedback, Requests, and Bugs
+
+Our process for submitting feedback, feature requests, and reporting bugs usually begins by discussion on [our chat](http://wiki.kiwix.org/wiki/Communication#Chat) and, after initial clarification, through [GitHub issues](https://help.github.com/articles/about-issues/). Each project repository generally maintains its own set of issues:
+
+        https://github.com/openzim/<repository-name>/issues
+
+Some projects have additional templates or sets of questions for each issue, which you will be prompted to fill out when creating one.
+
+Issues that span multiple projects or are about coordinating how we work overall are in the [Overview Issue Tracker](https://github.com/openzim/overview/issues).
+
+### Submitting Code and Documentation Changes
+
+We still do not have [project guidelines](./guidelines/project_guidelines.md) for all of the projects hosted in our [GitHub Organization](https://github.com/openzim), which new repositories should follow during their creation.
+
+Our process for accepting changes operates by [Pull Request (PR)](https://help.github.com/articles/about-pull-requests/) and has a few steps:
+
+1.  If you haven't submitted anything before, and you aren't (yet!) a member of our organization, **fork and clone** the repo:
+
+        $ git clone git@github.com:<your-username>/<repository-name>.git
+
+    Organization members should clone the upsteam repo, instead of working from a personal fork:
+
+        $ git clone git@github.com:openzim/<repository-name>.git
+
+1.  Create a **new branch** for the changes you want to work on. Choose a topic for your branch name that reflects the change:
+
+        $ git checkout -b <branch-name>
+
+1.  **Create or modify the files** with your changes. If you want to show other people work that isn't ready to merge in, commit your changes then create a pull request (PR) with _WIP_ or _Work In Progress_ in the title.
+
+        https://github.com/openzim/<repository-name>/pull/new/master
+
+1.  Once your changes are ready for final review, commit your changes then modify or **create your pull request (PR)**, assign as a reviewer or ping (using "`@<username>`") a Lieutenant (someone able to merge in PRs) active on the project (all Lieutenants can be pinged via `@openzim/lieutenants`)
+
+1.  Allow others sufficient **time for review and comments** before merging. We make use of GitHub's review feature to comment in-line on PRs when possible. There may be some fixes or adjustments you'll have to make based on feedback.
+
+1.  Once you have integrated comments, or waited for feedback, a Lieutenant should merge your changes in!
+
+_These guidelines are based on [Tools for Government Data Archiving](https://github.com/edgi-govdata-archiving/overview/blob/master/CONTRIBUTING.md)'s._