Update README.markdown with v1.2.3 notice.

README.markdown

+**Note, a version 1.2.3 PCB can be found in the v1.2.3 branch, but we are still testing it.
+This is a minor update that adds status LEDs and improves USB connection reliability.
+Older boards will still be supported.**
+
 # DStat Hardware
 This is the documentation for the DStat hardware.