Darwin: Add a README.md for the branch.

Just some top level information on this specific branch.

Current 12.2Dr0

Author: iains

README.md

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+# This is a branch of GCC 12-1 with experimental support for AArch64 (Arm64) on Darwin.
+
+The branch is recommended for (and has been tested on) powerpc, i686, x86_64 and aarch64 Darwin from Darwin9 (Mac OSX 10.5) through Darwin21 (macOS 12) (for architectures relevant to each version).  The branch has also been tested to build cross-compilers between x86_64 and aarch64 and on aarch64-linux-gnu without regressions.
+
+The support for Aarch64 (Arm64) remains experimental but is adequate now for many programs (and self-bootstrap).
+
+Please see README for general information on the GCC sources.
+
+Please see the GCC 12.1 release documentation for the implementation status of core language features.
+
+Please see gcc/config/aarch64/darwinpcs.md for a description of the macOS AArch64 (Arm64) ABI support.
+
+**Please report issues for this branch to:
+https://github.com/iains/gcc-12-branch/issues**
+
+**_The current release is GCC-12.2-darwin-r0. (August 2022)_**
+
+This release includes:
+ * All upstream fixes since 12.1.
+ * Improves the build to omit runpaths from the runtime libraries.
+ * A fix for PR106435 which improves the handling of cross-TU global initializers for thread-local variables.
+ * Fixes for builds and testing (include Ada) on Darwin8 (MacOSX 10.4, Tiger).
+
+**_GCC-12.1-darwin-r1. (June 2022)_**
+
+This release:
+ * Fixes a critical bug in the Arm64 port (related to Issue #3, which is unfixed in Xcode toolchains in common use).
+ * Adds a new configure option ```--with-darwin-extra-rpath=``` which allows a distribution to prepend a run path to the list automatically added by the compiler.  This is to allow for cases where the distribution refers to installed libraries using symlinks from outside the installation paths.
+ * Resolves a potential issue with library references where a minor GCC version is replaced by the next and the old version is deleted.
+ * Amends some of the system version processing to handle changes expected in macOS 13+.
+ * Improves interoperability with clang in mixed code projects.
+
+Thanks to contributors and testers.
+
+**_GCC-12.1-darwin-r0. (May 2022)_**
+
+Test results : https://github.com/iains/gcc-12-branch/issues/1#issuecomment-1120194463
+
+Thanks to:
+ * 'FX' (https://github.com/fxcoudert) for the main part of the ```__float128``` support, many test fixes and help in testing this release more widely.
+ * Andrew Burgess (while at Embecosm) for the work on heap based trampolines.
+ * Maxim Blinov  (Embecosm) for work on modifications to GCC's function lowering code to enable better support of the Darwin Arm64 ABI.
+
+Iain Sandoe.