Create CONTRIBUTING file

https://help.github.com/articles/setting-guidelines-for-repository-contributors/

CONTRIBUTING.md

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+Contributing to `qubes-doc`
+===========================
+
+Thank you for your interest in contributing to `qubes-doc`, the Qubes OS
+Project's dedicated documentation repository! Please take a moment to
+familiarize yourself with the terms defined in the [glossary], and try to use
+these terms consistently and accurately throughout your writing and editing.
+
+Please report all documentation issues in [qubes-issues]. 
+
+
+Markdown Conventions
+--------------------
+
+All the documentation is written in Markdown for maximum accessibility. When
+making contributions, please try to observe the following style conventions:
+
+ * Use spaces instead of tabs.
+ * Hard wrap Markdown lines at 80 characters.
+ * If appropriate, make numerals in numbered lists match between Markdown
+   source and HTML output.
+   * Rationale: In the event that a user is required to read the Markdown source
+     directly, this will make it easier to follow, e.g., numbered steps in a set
+     of instructions.
+ * Use hanging indentations  
+   where appropriate.
+ * Use underline headings (`=====` and `-----`) if possible. If this is not
+   possible, use Atx-style headings on both the left and right sides
+   (`### H3 ###`).
+ * Use `[reference-style][ref]` links.  
+ 
+`[ref]: https://daringfireball.net/projects/markdown/syntax#link`
+
+([This][md] is a great source for learning about Markdown.)
+
+
+Git Conventions
+---------------
+
+Please attempt to follow these conventions when writing your Git commit
+messages:
+
+ * Separate the subject line from the body with a blank line.
+ * Limit the subject line to approximately 50 characters.
+ * Capitalize the subject line.
+ * Do not end the subject line with a period.
+ * Use the imperative mood in the subject line.
+ * Wrap the body at 72 characters.
+ * Use the body to explain *what* and *why* rather than *how*.
+
+For details, examples, and the rationale behind each of these conventions,
+please see [this blog post][git-commit], which is the source of this list.
+
+
+[glossary]: https://www.qubes-os.org/doc/glossary/
+[qubes-issues]: https://github.com/QubesOS/qubes-issues/issues
+[md]: https://daringfireball.net/projects/markdown/
+[git-commit]: http://chris.beams.io/posts/git-commit/
+