changelog: updates for 4.2 release

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 This is a list of notable changes to Hyperscan, in reverse chronological order.
 
+## [4.2.0] 2016-05-31
+- Introduce an interpreter for many complex actions to replace the use of
+  internal reports within the core of Hyperscan (the "Rose" engine). This
+  improves scanning performance and reduces database size for many pattern
+  sets.
+- Many enhancements to the acceleration framework used by NFA and DFA engines,
+  including more flexible multibyte implementations and more AVX2 support. This
+  improves scanning performance for many pattern sets.
+- Improved prefiltering support for complex patterns containing very large
+  bounded repeats (`R{M,N}` with large `N`).
+- Improve scanning performance of pattern sets with a very large number of
+  EOD-anchored patterns.
+- Improve scanning performance of large pattern sets that use the
+  `HS_FLAG_SINGLEMATCH` flag.
+- Improve scanning performance of pattern sets that contain a single literal by
+  improving the "Noodle" literal matcher.
+- Small reductions in total stream state for many pattern sets.
+- Improve runtime detection of AVX2 support.
+- Disable -Werror for release builds, in order to behave better for packagers
+  and users with different compiler combinations than those that we test.
+- Improve support for building on Windows with MSVC 2015 (github issue #14).
+  Support for Hyperscan on Windows is still experimental.
+- Small updates to fix warnings identified by Coverity.
+- Remove Python codegen for the "FDR" and "Teddy" literal matchers. These are
+  now implemented directly in C code.
+- Remove the specialist "Sidecar" engine in favour of using our more general
+  repeat engines.
+- New API function: add the `hs_expression_ext_info()` function. This is a
+  variant of `hs_expression_info()` that can accept patterns with extended
+  parameters.
+- New API error value: add the `HS_SCRATCH_IN_USE` error, which is returned
+  when Hyperscan detects that a scratch region is already in use on entry to an
+  API function.
+
 ## [4.1.0] 2015-12-18
 - Update version of PCRE used by testing tools as a syntax and semantic
   reference to PCRE 8.38.