Create CONTRIBUTING.md

Based on:
* https://github.com/projectacrn/acrn-hypervisor/blob/master/doc/developer-guides/contribute_guidelines.rst

Many thanks to @AlexAltea for converting the first draft of this
document to Markdown format.

CONTRIBUTING.md

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+# Contribution Guidelines
+
+As an open-source project, we welcome and encourage the community to
+submit patches directly to project HAXM. In our collaborative open
+source environment, standards and methods for submitting changes help
+reduce the chaos that can result from an active development community.
+
+This document explains how to submit patches to the project so your
+patch will be accepted quickly in the codebase.
+
+## Licensing
+
+Licensing is very important to open source projects. It helps ensure the
+software continues to be available under the terms that the author
+desired.
+
+Project HAXM uses a BSD-3-Clause license, as found in the
+[LICENSE](https://github.com/intel/haxm/blob/master/LICENSE)
+in the project's GitHub repo.
+
+A license tells you what rights you have as a developer, as provided by
+the copyright holder. It is important that the contributor fully
+understands the licensing rights and agrees to them. Sometimes the
+copyright holder isn't the contributor, such as when the contributor is
+doing work on behalf of a company.
+
+## Developer Certification of Origin (DCO)
+
+To make a good faith effort to ensure licensing criteria are met,
+project HAXM requires the Developer Certificate of Origin (DCO) process
+to be followed.
+
+The DCO is an attestation attached to every contribution made by every
+developer. In the commit message of the contribution (described more
+fully later in this document), the developer simply adds a
+``Signed-off-by`` statement and thereby agrees to the DCO.
+
+When a developer submits a patch, it is a commitment that the
+contributor has the right to submit the patch per the license. The DCO
+agreement is shown below and at https://developercertificate.org.
+
+```
+    Developer's Certificate of Origin 1.1
+
+    By making a contribution to this project, I certify that:
+
+    (a) The contribution was created in whole or in part by me and I
+        have the right to submit it under the open source license
+        indicated in the file; or
+
+    (b) The contribution is based upon previous work that, to the
+        best of my knowledge, is covered under an appropriate open
+        source license and I have the right under that license to
+        submit that work with modifications, whether created in whole
+        or in part by me, under the same open source license (unless
+        I am permitted to submit under a different license), as
+        indicated in the file; or
+
+    (c) The contribution was provided directly to me by some other
+        person who certified (a), (b) or (c) and I have not modified
+        it.
+
+    (d) I understand and agree that this project and the contribution
+        are public and that a record of the contribution (including
+        all personal information I submit with it, including my
+        sign-off) is maintained indefinitely and may be redistributed
+        consistent with this project or the open source license(s)
+        involved.
+```
+
+### DCO Sign-Off Methods
+
+The DCO requires that a sign-off message, in the following format,
+appears on each commit in the pull request:
+
+`Signed-off-by: Haxmus Jones <[email protected]>`
+
+You are required to use your real name in the sign-off message.
+
+The DCO text can either be manually added to your commit body, or you can add
+either ``-s`` or ``--signoff`` to your usual Git commit commands. If you forget
+to add the sign-off you can also amend a previous commit with the sign-off by
+running ``git commit --amend -s``. If you've pushed your changes to GitHub
+already you'll need to force push your branch after this with ``git push -f``.
+
+
+**Note:**
+
+The name and email address of the account you use to submit your PR must
+match the name and email address on the ``Signed-off-by`` line in
+your commit message.