Add release notes about the unroll pragmas.

New Pragmas in Clang

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Clang now supports the ...

Clang now supports the `#pragma unroll` and `#pragma nounroll` directives to

specify loop unrolling optimization hints.  Placed just prior to the desired

loop, `#pragma unroll` directs the loop unroller to attempt to fully unroll the

loop.  The pragma may also be specified with a positive integer parameter

indicating the desired unroll count: `#pragma unroll _value_`.  The unroll count

parameter can be optionally enclosed in parentheses. The directive `#pragma

nounroll` indicates that the loop should not be unrolled.  These unrolling hints

may also be expressed using the `#pragma clang loop` directive.  See the Clang

`language extensions

<http://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations>`_

for details.

Windows Support

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