NUVOTON: Add openocd upload method document

This adds document to enable openocd upload method for Nuvoton
NuMicro Cortex-M series.

targets/TARGET_NUVOTON/README.md

@@ -0,0 +1,15 @@
+# Nuvoton NuMicro Cortex-M series
+
+## Nuvoton fork of OpenOCD
+
+Nuvoton NuMicro Cortex-M support for openocd is forked
+[here](https://github.com/OpenNuvoton/OpenOCD-Nuvoton),
+but has not yet upstreamed.
+To enable openocd upload method for the series,
+install [NuEclipse IDE](https://github.com/OpenNuvoton/Nuvoton_Tools)
+which will also install the necessary openocd.
+
+> **ℹ️ Information**
+>
+> On Windows, openocd will usually install at:
+`C:\Program Files (x86)\Nuvoton Tools\OpenOCD\bin\openocd.exe`.