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.Windows.md

@@ -0,0 +1,44 @@
+Building OpenOCD for Windows
+----------------------------
+
+You can build OpenOCD for Windows natively with either MinGW-w64/MSYS
+or Cygwin (plain MinGW might work with --disable-werror but is not
+recommended as it doesn't provide enough C99 compatibility).
+Alternatively, one can cross-compile it using MinGW-w64 on a *nix
+host. See README.md for the generic instructions.
+
+Also, the MSYS2 project provides both ready-made binaries and an easy
+way to self-compile from their software repository out of the box.
+
+USB adapters
+------------
+
+For the adapters that use a HID-based protocol, e.g. CMSIS-DAP, you do
+not need to perform any additional configuration.
+
+For all the others you usually need to have WinUSB.sys (or
+libusbK.sys) driver installed. Some vendor software (e.g. for
+ST-LINKv2) does it on its own. For the other cases the easiest way to
+assign WinUSB to a device is to use the latest Zadig installer:
+
+  http://zadig.akeo.ie
+
+When using a composite USB device, it's often necessary to assign
+WinUSB.sys to the composite parent instead of the specific
+interface. To do that one needs to activate an advanced option in the
+Zadig installer.
+
+If you need to use the same adapter with other applications that may
+require another driver, a solution for Windows Vista and above is to
+activate the IgnoreHWSerNum registry setting for the USB device.
+
+That setting forces Windows to associate the driver per port instead of
+per serial number, the same behaviour as when the device does not contain
+a serial number. So different drivers can be installed for the adapter on
+different ports and you just need to plug the adapter into the correct
+port depending on which application to use.
+
+For more information, see:
+
+  https://learn.microsoft.com/en-us/windows-hardware/drivers/usbcon/usb-device-specific-registry-settings
+  http://www.ftdichip.com/Support/Knowledgebase/index.html?ignorehardwareserialnumber.htm