Latest abrok Development Version May 2nd 2021 (SF The gift that keeps on giving )

!! latest version !!

Windows x64 for Haswell CPUs
Windows x64 for modern computers + AVX2
Windows x64 for modern computers
Windows x64 + SSSE3
Windows x64
Windows 32
Linux x64 for Haswell CPUs
Linux x64 for modern computers + AVX2
Linux x64 for modern computers
Linux x64 + SSSE3
Linux x64

Author: Unai Corzo
Date: Sun May 2 17:48:57 2021 +0200
Timestamp: 1619970537

Simplify check extension

Simplify check extension, as it seems not to bring any strength and thus is no longer needed.

STC https://tests.stockfishchess.org/tests/view/608c18e995e7f1852abd2b81
LLR: 2.94 (-2.94,2.94) <-2.50,0.50>
Total: 54544 W: 4891 L: 4815 D: 44838 Elo +0.48
Ptnml(0-2): 186, 3889, 19081, 3895, 221

LTC https://tests.stockfishchess.org/tests/view/608c6ab195e7f1852abd2bc6
LLR: 2.95 (-2.94,2.94) <-2.50,0.50>
Total: 51008 W: 1845 L: 1794 D: 47369 Elo +0.35
Ptnml(0-2): 31, 1591, 22206, 1648, 28

closes https://github.com/official-stockfish/Stockfish/pull/3452

bench: 3993071
see source

Author: Joost VandeVondele
Date: Sun May 2 17:43:02 2021 +0200
Timestamp: 1619970182

Add some more information on the UCI protocol

Improve README.md: provide a link to the protocol,
and document some non-standard options.

fixes https://github.com/official-stockfish/Stockfish/issues/3446

closes https://github.com/official-stockfish/Stockfish/pull/3450

No functional change
see source

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