Copy ANNOUNCE for 2.1 release.

Author: davidchisnall

ANNOUNCE.2.1

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+GNUstep Objective-C Runtime 2.1
+=================================
+
+This is the first update to the second major release of the GNUstep Objective-C
+runtime (a.k.a.  libobjc2).  This runtime was designed to support the features
+of modern dialects of Objective-C for use with GNUstep and other Objective-C
+programs.
+
+*NOTE:* This is the first release to use submodules.  If you are downloading
+the sources from git, please make sure that you do a recursive clone.  If you
+forget, the build system will give you instructions to correct this.
+Tarballs from GitHub do not include submodules, so if you are downloading the
+tarball then you will need to download the submodule separately.
+
+
+Highlights of this release include:
+
+- Numerous improvements to the Objective-C++ exception interoperation code.
+  The runtime now dynamically detects whether the libcxxrt or libsupc++ variant
+  of the Itanium C++ Exception ABI is being used
+
+- Sending a message to `super` where the corresponding method did not exist was
+  silently ignored in previous versions of the runtime.  This now correctly
+  invokes the forwarding hooks and so (with an implementation of the Foundation
+  framework, such as GNUstep Base or WinObjC) will trigger an exception or
+  invoke `forwardInvocation:`.
+
+- The checks for overloaded memory management methods were incorrect, causing
+  some classes to be incorrectly opted into ARC fast paths.  These checks are
+  now correct.
+
+- Several memory management bugs in corner cases of weak reference management
+  were fixed.
+
+- The ARM assembly implementation of `objc_msgSend` now correctly restores the
+  stack after calling a forwarding implementation.  This bug caused stack
+  corruption and usually crashing on ARM.
+
+- The ARC code has been rewritten as C++, using a well-tested third-party
+  Robin-Hood hash table to store weak references, replacing the home-grown
+  version.  This improves performance and reduces the likelihood of bugs
+  arising from the hash table implementation.
+
+- Control Flow Guard (CGF) checks were added on Windows on x86 (32- and
+  64-bit).  If Objective-C code is compiled with CFG enabled then
+  `objc_msgSend` will crash if it attempts to jump to an address that is not a
+  valid function entry point.
+
+- The function signatures in the blocks headers were updated for compatibility
+  with recent macOS releases.
+
+- Support for the C11 _Atomic type qualifier in property metadata was added.
+
+You may obtain the code for this release from git and use the 2.1 branch:
+
+https://github.com/gnustep/libobjc2.git
+
+Alternatively, a tarball is available from:
+
+https://github.com/gnustep/libobjc2/archive/v2.1.zip
+https://github.com/gnustep/libobjc2/archive/v2.1.tar.gz
+
+The submodule is available from:
+
+https://github.com/Tessil/robin-map/archive/757de82.zip
+https://github.com/Tessil/robin-map/archive/757de82.tar.gz
+
+This will extract as robin-map-757de829927489bee55ab02147484850c687b620.
+You must move the contents of that directory into third_party/robin_map in the
+libobjc2 tree.
+
+
+The runtime library is responsible for implementing the core features of the
+object model, as well as exposing introspection features to the user.  The
+GNUstep runtime implements a superset of Apple's Objective-C Runtime APIs.
+
+If you come across any problems, please file them in the issue tracker:
+
+https://github.com/gnustep/libobjc2/issues