libbacktrace: update README

Author: ianlancetaylor

README.md

@@ -10,6 +10,8 @@ The libbacktrace library may be linked into a program or library and
 used to produce symbolic backtraces.
 Sample uses would be to print a detailed backtrace when an error
 occurs or to gather detailed profiling information.
+In general the functions provided by this library are async-signal-safe,
+meaning that they may be safely called from a signal handler.
 
 The libbacktrace library is provided under a BSD license.
 See the source files for the exact license text.
@@ -23,11 +25,12 @@ will work.
 See the source file backtrace-supported.h.in for the macros that it
 defines.
 
-As of January 2018, libbacktrace only supports ELF, PE/COFF, and XCOFF
-executables with DWARF debugging information.
+As of October 2020, libbacktrace supports ELF, PE/COFF, Mach-O, and
+XCOFF executables with DWARF debugging information.
+In other words, it supports GNU/Linux, *BSD, macOS, Windows, and AIX.
 The library is written to make it straightforward to add support for
 other object file and debugging formats.
 
 The library relies on the C++ unwind API defined at
 https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html
-This API is provided by GCC.
+This API is provided by GCC and clang.