Quality: Adding document that describes how to contribute.

CONTRIBUTING.md

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+# Contributing to Nuitka
+
+This document aims to give an overview of how to contribute to Nuitka. It tries
+to answer commonly asked questions regarding that, and to provide some insight
+
+* If you plan on submitting an issue, please follow this
+  [template](https://github.com/kayhayen/Nuitka/blob/master/.github/ISSUE_TEMPLATE.md).
+* If you want to open a pull request make sure to read the information on this
+  page but also have a look at our
+  [pull request template](https://github.com/kayhayen/Nuitka/blob/master/.github/PULL_REQUEST_TEMPLATE.md).
+
+## Getting Started
+
+ * Read the [Nuitka User Manual](http://nuitka.net/doc/user-manual.html)
+ * Read the [Nuitka Developer Manual](http://nuitka.net/doc/developer-manual.html)
+ * Checkout the git repo of Nuitka
+
+## Submitting a Pull Request
+
+Pull requests are great. Here's a quick guide:
+
+First, if the PR is directly related to an already existing issue (which has no
+PR yet), drop a note in that issue before opening the PR.
+
+ 1. Fork the repo on github.
+ 2. Make a new branch. For feature additions/changes base your new branch at
+    `develop`, for pure bugfixes base your new branch at e.g. `master`.
+ 3. Ideally add a test specific for your change to demonstrate it. Due to Nuitka
+    testing being basically to compile the whole world, it's ok to not have one.
+    But obviously if you fix something, it wasn't observed by that, it would be
+    good to provide a reproducer.
+ 4. Make the tests pass.
+ 5. Push to your fork and submit a pull request.
+    - for feature branches set base branch to `nuitka:develop`
+    - for bugfix branches set base branch to the latests maintenance branch, e.g. `nuitka:master`
+ 6. Wait for review, suggestions for improvements or alternative ideas may
+    happen. Keep in mind that PR checklist items can be met after the pull
+    request has been opened by adding more commits to the branch.
+
+All the submitted pieces including potential data must be compatible with the
+Apache License 2, which already says that you sending source, e.g. via pull
+request, license it as that too.
+
+## Submitting an Issue
+
+If you want to ask a question about a specific Nuitka aspect, please first of all..
+
+ * Search through [Github issues tagged as "question"](https://github.com/kayhayen/Nuitka/issues?q=label%3Aquestion)
+ * Search the [nuitka-dev] mailing list archives, e.g.
+   searching for term "standalone macos" via Google with search string "standalone site:https://www.freelists.org/archive may already reveal what you are looking for
+ * If asked in neither forum, ask it there.
+
+If you want to post a problem/bug, to help us understand and resolve your issue
+please check that you have provided the information below:
+
+*  Nuitka version, full Python version and Platform (Windows, OSX, Linux ...)
+*  How did you install Nuitka and Python (pip, anaconda, deb, rpm, from source,
+   what is a virtualenv ...), this is very important usually.
+*  If possible please supply a [Short, Self Contained, Correct, Example](http://sscce.org/)
+      that demonstrates the issue i.e a small piece of code which reproduces
+      the issue and can be run with out any other (or as few as possible)
+      external dependencies.
+*  If this is a regression (used to work in an earlier version of Nuitka),
+   please note what you know about that.
+
+You can also do a quick check whether..
+
+ * the bug was already fixed in the branch for the next release, e.g. for the
+   `develop` branch pre-release packages out.
+
+ * if it was already reported and/or is maybe even being worked on already by
+   checking open issues of the corresponding milestone, e.g. see
+
+   * [open and closed issues with "bug" label](
+      https://github.com/kayhayen/Nuitka/issues?q=label%3Abug+).
+
+## And finally
+
+Welcome on board. Nuitka is an ambitious project. Join it now.