libbacktrace: add notes about dl_iterate_phdr to README

	* README: Add notes about dl_iterate_phdr.

For #20

README.md

@@ -10,8 +10,18 @@ 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.
+That said, on systems that use `dl_iterate_phdr`, such as GNU/Linux,
+gitthe first call to a libbacktrace function will call `dl_iterate_phdr`,
+which is not in general async-signal-safe.  Therefore, programs
+that call libbacktrace from a signal handler should ensure that they
+make an initial call from outside of a signal handler.
+Similar considerations apply when arranging to call libbacktrace
+from within malloc; `dl_iterate_phdr` can also call malloc,
+so make an initial call to a libbacktrace function outside of
+malloc before trying to call libbacktrace functions within malloc.
 
 The libbacktrace library is provided under a BSD license.
 See the source files for the exact license text.
@@ -25,7 +35,7 @@ will work.
 See the source file backtrace-supported.h.in for the macros that it
 defines.
 
-As of October 2020, libbacktrace supports ELF, PE/COFF, Mach-O, and
+As of July 2024, 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