Add CONTRIBUTING guidelines

CONTRIBUTING.md

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+# CONTRIBUTING GUIDELINES
+
+Oh-My-Bash is a community-driven project. Contribution is welcome, encouraged and appreciated.
+It is also essential for the development of the project.
+
+These guidelines are an attempt at better addressing the huge amount of pending
+issues and pull requests. Please read them closely.
+
+Foremost, be so kind as to [search](#use-the-search-luke). This ensures any contribution
+you would make is not already covered.
+
+* [Issues](#reporting-issues)
+  * [You have a problem](#you-have-a-problem)
+  * [You have a suggestion](#you-have-a-suggestion)
+* [Pull Requests](#submitting-pull-requests)
+  * [Getting started](#getting-started)
+  * [You have a solution](#you-have-a-solution)
+  * [You have an addition](#you-have-an-addition)
+* [Information sources (_aka_ search)](#use-the-search-luke)
+
+**BONUS:** [Volunteering](#you-have-spare-time-to-volunteer)
+
+## Reporting Issues
+
+### You have a problem
+
+Please be so kind as to [search](#use-the-search-luke) for any open issue already covering
+your problem.
+
+If you find one, comment on it so we can know there are more people experiencing it.
+
+If not, look at the [Troubleshooting](https://github.com/ohmybash/oh-my-bash/wiki/Troubleshooting)
+page for instructions on how to gather data to better debug your problem.
+
+Then, you can go ahead and create an issue with as much detail as you can provide.
+It should include the data gathered as indicated above, along with:
+
+1. How to reproduce the problem
+2. What the correct behavior should be
+3. What the actual behavior is
+
+Please copy to anyone relevant (_eg_ plugin maintainers) by mentioning their GitHub handle
+(starting with `@`) in your message.
+
+We will do our very best to help you.
+
+### You have a suggestion
+
+Please be so kind as to [search](#use-the-search-luke) for any open issue already covering
+your suggestion.
+
+If you find one, comment on it so we can know there are more people supporting it.
+
+If not, you can go ahead and create an issue. Please copy to anyone relevant (_eg_ plugin
+maintainers) by mentioning their GitHub handle (starting with `@`) in your message.
+
+## Submitting Pull Requests
+
+### Getting started
+
+You should be familiar with the basics of
+[contributing on GitHub](https://help.github.com/articles/using-pull-requests) and have a fork
+[properly set up](https://github.com/ohmybash/oh-my-bash/wiki/Contribution-Technical-Practices).
+
+You MUST always create PRs with _a dedicated branch_ based on the latest upstream tree.
+
+If you create your own PR, please make sure you do it right. Also be so kind as to reference
+any issue that would be solved in the PR description body,
+[for instance](https://help.github.com/articles/closing-issues-via-commit-messages/)
+_"Fixes #XXXX"_ for issue number XXXX.
+
+### You have a solution
+
+Please be so kind as to [search](#use-the-search-luke) for any open issue already covering
+your [problem](#you-have-a-problem), and any pending/merged/rejected PR covering your solution.
+
+If the solution is already reported, try it out and +1 the pull request if the
+solution works ok. On the other hand, if you think your solution is better, post
+it with a reference to the other one so we can have both solutions to compare.
+
+If not, then go ahead and submit a PR. Please copy to anyone relevant (e.g. plugin
+maintainers) by mentioning their GitHub handle (starting with `@`) in your message.
+
+### You have an addition
+
+Please [do not](https://github.com/ohmybash/oh-my-bash/wiki/Themes#dont-send-us-your-theme-for-now)
+send themes for now.
+
+Please be so kind as to [search](#use-the-search-luke) for any pending, merged or rejected Pull Requests
+covering or related to what you want to add.
+
+If you find one, try it out and work with the author on a common solution.
+
+If not, then go ahead and submit a PR. Please copy to anyone relevant (_eg_ plugin
+maintainers) by mentioning their GitHub handle (starting with `@`) in your message.
+
+For any extensive change, _eg_ a new plugin, you will have to find testers to +1 your PR.
+
+----
+
+## Use the Search, Luke
+
+_May the Force (of past experiences) be with you_
+
+GitHub offers [many search features](https://help.github.com/articles/searching-github/)
+to help you check whether a similar contribution to yours already exists. Please search
+before making any contribution, it avoids duplicates and eases maintenance. Trust me,
+that works 90% of the time.
+
+You can also take a look at the [FAQ](https://github.com/ohmybash/oh-my-bash/wiki/FAQ)
+to be sure your contribution has not already come up.
+
+If all fails, your thing has probably not been reported yet, so you can go ahead
+and [create an issue](#reporting-issues) or [submit a PR](#submitting-pull-requests).
+
+----
+
+### You have spare time to volunteer
+
+Very nice!! :)
+
+Please have a look at the [Volunteer](https://github.com/ohmybash/oh-my-bash/wiki/Volunteers)
+page for instructions on where to start and more.