Create README.md

Author: davidad

README.md

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+I'm switching from [`nasm`](http://www.nasm.us/) to [`fasm`](http://flatassembler.net/), because the latter has
+a more expressive macro facility. The downside of `fasm` is that it doesn't support OSX. But, with some Google-fu and Makefile-fu,
+I have made it support OSX, with the same level of portability and convenience that I had previously achieved
+with `nasm`! (The key insights were found in [this forum thread](http://board.flatassembler.net/topic.php?t=13413).)
+
+In order to use the Makefile here, you will need [`objconv`](http://www.agner.org/optimize/#objconv), and you will also
+need an already-working `fasm`. If you're on OSX, that might be quite difficult, so I'm including a working OSX
+`fasm` binary as a download in this repository.
+
+Verify that you have these installed:
+
+    $ fasm
+    flat assembler  version 1.71.17
+    usage: fasm <source> [output]
+    $ objconv
+    
+    Object file converter version 2.32 for x86 and x86-64 platforms.
+    Copyright (c) 2013 by Agner Fog. Gnu General Public License.
+
+Then, it's as simple as
+
+```ShellSession
+$ git clone https://github.com/davidad/fasm_example_64.git
+Cloning into 'fasm_example_64'...
+remote: Counting objects: 13, done.
+remote: Compressing objects: 100% (10/10), done.
+remote: Total 13 (delta 3), reused 13 (delta 3)
+Unpacking objects: 100% (13/13), done.
+$ cd fasm_example_64
+$ make
+ln -s osx.inc syscalls.inc
+fasm hello64.asm hello64.o
+flat assembler  version 1.71.17  (16384 kilobytes memory)
+2 passes, 467 bytes.
+objconv -fmacho -ar:start:_start -nu hello64.o hello64.mach.o
+
+Input file: hello64.o, output file: hello64.mach.o
+Converting from ELF64 to Mach-O Little Endian64
+Adding leading underscores to symbol names
+
+  0 Debug sections removed
+  0 Exception sections removed
+  0 Changes in leading underscores on symbol names
+  1 Symbol names changed
+  1 Public symbol names aliased
+ld hello64.mach.o -o hello64.mach.exe
+ld: warning: -macosx_version_min not specified, assuming 10.6
+ln -f hello64.mach.exe hello64
+rm hello64.o hello64.mach.exe hello64.mach.o
+$ ./hello64
+Hello world!
+```