Add retroactive W3C license, notices and contribution terms

CONTRIBUTING.md

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+# Contributing to web-map-doc
+
+This repository is the documentation for the [MapML.js](https://github.com/Maps4HTML/MapML.js)
+polyfill implementation of [Map Markup Language](https://maps4html.org/MapML-Specification/spec/).
+
+It is used for work in the [W3C Maps For HTML Community Group](http://www.w3.org/community/maps4html/),
+governed by the [W3C Community Contributor License Agreement (CLA)](http://www.w3.org/community/about/agreements/cla/).
+
+**Everyone who contributes to this repository must be a CLA signatory.** Join the
+Community Group and sign the CLA before opening a pull request; contributions from
+non-signatories cannot be accepted.
+
+By contributing to this repository, you license your contribution to the
+Community Group and to the public under the
+[W3C Software and Document License](http://www.w3.org/Consortium/Legal/2015/copyright-software-and-document)
+(`W3C-20150513`), as set out in [LICENSE.md](./LICENSE.md). This applies to
+documentation, example code, demonstration pages and assets alike. You retain
+copyright in your contribution.
+
+If you are not the sole contributor to a contribution (pull request), please identify all
+contributors in the pull request comment.
+
+To add a contributor (other than yourself, that's automatic), mark them one per line as follows:
+
+```
++@github_username
+```
+
+If you added a contributor by mistake, you can remove them in a comment with:
+
+```
+-@github_username
+```
+
+If you are making a pull request on behalf of someone else but you had no part in designing the
+feature, you can remove yourself with the above syntax.
+
+## Third-party material
+
+Do not add third-party code or assets to this repository without recording the
+required notices in [NOTICE](./NOTICE). The `static/dist/` directory is a
+synchronized copy of the MapML.js build and should not be edited here.
+
+If you have suggestions or want to open a pull request for improved documentation,
+it's a good idea to open an issue and discuss with the community in advance.

LICENSE.md

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+All Reports in this Repository are licensed by Contributors under the
+[W3C Software and Document License](http://www.w3.org/Consortium/Legal/2015/copyright-software-and-document).
+Contributions to Specifications are made under the
+[W3C CLA](https://www.w3.org/community/about/agreements/cla/).
+
+Copyright 2021-2026 Canada Centre for Mapping and Earth Observation - Natural Resources Canada,
+and Malvoz <https://github.com/Malvoz>, together with the other contributors
+to this repository
+<https://github.com/Maps4HTML/web-map-doc/graphs/contributors>.
+
+SPDX-License-Identifier: W3C-20150513
+
+The work in this repository has been authored predominantly by employees of
+Natural Resources Canada.  Malvoz is the principal contributor from outside that group. 
+
+This license statement is published retroactively. It records the terms under
+which contributions to this repository have been made since its creation in
+February 2021, as a repository of the
+[W3C Maps For HTML Community Group](http://www.w3.org/community/maps4html/).
+Everyone who contributes to this repository must be a signatory of the
+[W3C Community Contributor License Agreement](http://www.w3.org/community/about/agreements/cla/),
+and every contribution is made under that agreement. This license applies to
+the entire history of the repository, including all documentation, example
+code, demonstration pages and assets authored here.
+
+This repository also redistributes third-party material, including a build of
+[MapML.js](https://github.com/Maps4HTML/MapML.js) under `static/dist/`, which
+carries its own licenses. Those works remain governed by their own terms, which
+the license below does not supersede. The required notices, and the full text
+of those licenses, are in the [NOTICE](./NOTICE) file. You must retain the
+`NOTICE` file when redistributing this work, with or without modification.
+
+License
+
+By obtaining and/or copying this work, you (the licensee) agree that you have
+read, understood, and will comply with the following terms and conditions.
+
+Permission to copy, modify, and distribute this work, with or without
+modification, for any purpose and without fee or royalty is hereby granted,
+provided that you include the following on ALL copies of the work or portions
+thereof, including modifications:
+
+The full text of this NOTICE in a location viewable to users of the
+redistributed or derivative work.
+
+Any pre-existing intellectual property disclaimers, notices, or terms and
+conditions. If none exist, the W3C Software and Document Short Notice should
+be included.
+
+Notice of any changes or modifications, through a copyright statement on the
+new code or document such as "This software or document includes material
+copied from or derived from [title and URI of the W3C document].
+Copyright © [YEAR] W3C® (MIT, ERCIM, Keio, Beihang)."
+
+Disclaimers
+
+THIS WORK IS PROVIDED "AS IS," AND COPYRIGHT HOLDERS MAKE NO REPRESENTATIONS
+OR WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO, WARRANTIES OF
+MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE OR THAT THE USE OF THE
+SOFTWARE OR DOCUMENT WILL NOT INFRINGE ANY THIRD PARTY PATENTS, COPYRIGHTS,
+TRADEMARKS OR OTHER RIGHTS.
+COPYRIGHT HOLDERS WILL NOT BE LIABLE FOR ANY DIRECT, INDIRECT, SPECIAL OR
+CONSEQUENTIAL DAMAGES ARISING OUT OF ANY USE OF THE SOFTWARE OR DOCUMENT.
+
+The name and trademarks of copyright holders may NOT be used in advertising or
+publicity pertaining to the work without specific, written prior permission.
+Title to copyright in this work will at all times remain with copyright holders.