doc: Minor manual fix.

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Update license information

* LICENSES: New.
* Makefile.am (EXTRA_DIST): Add LICENSES.
* AUTHORS: Add list of copyright holders.
* README: Reference AUTHORS.

Signed-off-by: Werner Koch <wk@gnupg.org>
Add missing register clearing in to SHA-256 and SHA-512 assembly

* cipher/sha256-ssse3-amd64.S: Clear used XMM/YMM registers at return.
* cipher/sha512-avx-amd64.S: Ditto.
* cipher/sha512-avx2-bmi2-amd64.S: Ditto.
* cipher/sha512-ssse3-amd64.S: Ditto.
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Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>