README: Use Markdown extension

Ensures proper rendering on GitHub [1] / GitLab, clarifies that Markdown
syntax is expected from contributors, and enables editor syntax
highlighting.

[1] https://github.com/openocd-org/openocd

Change-Id: Icaff52ed2bf7d6f32b5812b5aff1c081e8b7507a
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/9388
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

README.macOS.md

@@ -0,0 +1,54 @@
+Building OpenOCD for macOS
+--------------------------
+
+There are a few prerequisites you will need first:
+
+- Xcode (install from the AppStore)
+- Command Line Tools (install from Xcode -> Preferences -> Downloads)
+- Gentoo Prefix (http://www.gentoo.org/proj/en/gentoo-alt/prefix/bootstrap.xml)
+  or
+- Homebrew (http://mxcl.github.io/homebrew/)
+  or
+- MacPorts (http://www.macports.org/install.php)
+
+
+If you're building manually you need Texinfo version 5.0 or later. The
+simplest way to get it is to use Homebrew (brew install texinfo) and
+then ``export PATH=/usr/local/opt/texinfo/bin:$PATH``.
+
+
+With Gentoo Prefix you can build the release version or the latest
+devel version (-9999) the usual way described in the Gentoo
+documentation. Alternatively, install the prerequisites and build
+manually from the sources.
+
+
+With Homebrew you can either run:
+  brew install [--HEAD] openocd (where optional --HEAD asks brew to
+                                 install the current git version)
+    or
+  brew install libtool automake libusb [hidapi] [libftdi]
+    (to install the needed dependencies and then proceed with the
+     manual building procedure)
+
+
+For building with MacPorts you need to run:
+  sudo port install libtool automake autoconf pkgconfig \
+    libusb [libftdi1]
+
+You should also specify LDFLAGS and CPPFLAGS to allow configure to use
+MacPorts' libraries, so run configure like this:
+  LDFLAGS=-L/opt/local/lib CPPFLAGS=-I/opt/local/include ./configure [options]
+
+
+See README.md for the generic building instructions.
+
+If you're using a USB adapter and have a driver kext matched to it,
+you will need to unload it prior to running OpenOCD. E.g. with Apple
+driver (OS X 10.9 or later) for FTDI run:
+  sudo kextunload -b com.apple.driver.AppleUSBFTDI
+for FTDI vendor driver use:
+  sudo kextunload FTDIUSBSerialDriver.kext
+
+To learn more on the topic please refer to the official libusb FAQ:
+https://github.com/libusb/libusb/wiki/FAQ