docs: Add note for building with MSVC

[khuiqel]

Problem

Due to Snappy's non-standard microarchitecture detection, SNAPPY_HAVE_SSSE3, SNAPPY_HAVE_X86_CRC32, and SNAPPY_HAVE_BMI2 will always be true on MSVC and therefore cause illegal instructions to be produced. For example, _bzhi_u32 in snappy::ExtractLowBytes().

Solution

A full solution would be far too invasive and beyond the scope of this PR, so the temporary solution is to update the README's compilation instructions to inform users of this problem. They can solve this issue by setting SNAPPY_HAVE_SSSE3, SNAPPY_HAVE_X86_CRC32, SNAPPY_HAVE_BMI2, SNAPPY_HAVE_NEON_CRC32, and SNAPPY_HAVE_NEON in their buildsystem while a proper solution is worked on.

For instance, a CPU without BMI2 (but with SSSE3 and CRC32) will fail the BM_UFlat benchmarks and the following tests:

[  FAILED  ] 14 tests, listed below:
[  FAILED  ] CorruptedTest.VerifyCorrupted
[  FAILED  ] Snappy.SimpleTests
[  FAILED  ] Snappy.AppendSelfPatternExtensionEdgeCases
[  FAILED  ] Snappy.AppendSelfPatternExtensionEdgeCasesExhaustive
[  FAILED  ] Snappy.MaxBlowup
[  FAILED  ] Snappy.RandomData
[  FAILED  ] Snappy.CompressionContext
[  FAILED  ] Snappy.CompressionContextStaticWorkspace
[  FAILED  ] Snappy.IOVecSinkEdgeCases
[  FAILED  ] Snappy.IOVecCopyOverflow
[  FAILED  ] Snappy.ZeroOffsetCopy
[  FAILED  ] Snappy.ZeroOffsetCopyValidation
[  FAILED  ] Snappy.LiteralLengthU32Overflow
[  FAILED  ] Snappy.TestBenchmarkFiles