docs: license the code under Apache 2.0 and the spec under CC BY-SA 4.0

COPYING.md

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+# Licensing
+
+Two licenses, because this repository holds two different kinds of thing.
+
+**The code is Apache License 2.0.** See [LICENSE](LICENSE). That covers the
+validator and directory service under `mews/`, `mewslint.py`, `md2mews.py`,
+`dtd/check_dtd.py`, the Pelican plugin under `pelican-mews/`, the packaging,
+the tests, and the pages of the mews.page site itself under `site/`.
+
+**The specification is Creative Commons Attribution-ShareAlike 4.0
+International.** See [doc/LICENSE](doc/LICENSE). That covers `doc/SPEC.md` and
+the other prose in `doc/`. Share it, translate it, build on it; say where it
+came from and pass the same terms on.
+
+**The default stylesheet, `mews-0.1.css`, is Apache 2.0**, even though section
+5.1 of the specification prints it in full. Section 5.2 asks every site to host
+its own copy, and nobody should have to wonder whether that puts a share-alike
+condition on the pages it styles.
+
+## Third-party files
+
+`dtd/xhtml-mobile12.dtd` is not ours and is not under either license. It is the
+XHTML Mobile Profile 1.2 driver DTD, Copyright 2008 Open Mobile Alliance Ltd.,
+all rights reserved, included here under the terms printed in its own header.
+It is kept in the repository for reference, as section 9.1 describes.
+
+`dtd/mews-0.1.dtd` is written by hand, but it describes a profile the Open
+Mobile Alliance defined, and element and attribute names are taken from the
+W3C's XHTML modularization. The file is Apache 2.0; what it describes is not
+ours to license.