add COPYING file

COPYING.md

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+COPYING.md for the KhronosGroup/OpenXR-Docs project
+===================================================
+
+# Licenses
+
+The OpenXR-Docs project uses several licenses.
+
+* The source files (in asciidoctor and other formats) for the OpenXR
+  Specification, reference pages, and supporting documentation are licensed
+  under the Creative Commons Attribution 4.0 International License
+  ("CC-BY").
+* Header files, scripts, programs, XML files, and other tooling used or
+  generated as part of the build process is licensed under the Apache
+  License, Version 2.0.
+* Some generated, transient files produced during the course of building the
+  specification, headers, or other targets may not have copyrights. These
+  are typically very short asciidoc fragments describing parts of the OpenXR
+  API, and are incorporated by reference into specification or reference
+  page builds.
+
+Users outside Khronos who create and post OpenXR Specifications, whether
+modified or not, should use the CC-BY license on the output documents (HTML,
+PDF, etc.) they generate.
+
+
+# Frequently Asked Questions
+
+Q: Why are the HTML and PDF Specifications posted on Khronos' website under
+a license which is neither CC-BY nor Apache 2.0?
+
+A: The Specifications posted by Khronos in the OpenXR Registry are licensed
+under the proprietary Khronos Specification License. Only these
+Specifications are Ratified by the Khronos Board of Promoters, and therefore
+they are the only Specifications covered by the Khronos Intellectual
+Property Rights Policy.
+
+
+Q: Does Khronos allow the creation and distribution of modified versions of
+the OpenXR Specification, such as translations to other languages?
+
+A: Yes. Such modified Specifications, since they are not created by Khronos,
+should be placed under the CC-BY license. If you believe your modifications
+are of general interest, consider contributing them back by making a pull
+request (PR) on the OpenXR-Docs project.
+
+
+Q: Can I contribute changes to the OpenXR Specification?
+
+A: Yes, by opening a PR on the KhronosGroup/OpenXR-Docs Github project. You
+must execute a click-through Contributor License Agreement, which brings
+your changes under the umbrella of the Khronos IP policy. We welcome
+feedback and proposed changes, but will not necessarily accept all such
+changes. Please keep PRs focused on solving a single problem; more ambitious
+PRs that try to solve multiple problems, or touch many parts of the
+Specification at once, are difficult for the OpenXR Working Group to review
+in a timely fashion.
+
+
+Q: Can you change the license on your files so they're compatible with my
+license?
+
+A: We've added an "Exceptions to the Apache 2.0 License" clause to the
+copyright on xr.xml, to make it more compatible with GPL-licensed software,
+such as externally-generated language bindings. This seems to have addressed
+the problems we're aware of. It is possible we could extend the Exception
+Clause to other Apache-licensed files in the project, or otherwise
+accommodate new needs of external projects; but working with the different
+Khronos member company IP lawyers to make license changes is a very slow
+process, constrained by the Khronos Member Agreement and IP Policy as well
+as by individual company concerns about their IP. Do not expect rapid
+changes in anything having to do with copyrights and licensing.