Added LED description table to README

(cherry picked from commit 7508afdb55)

README.md

@@ -169,3 +169,14 @@ configured voltage, the Sink will negotiate it, then turn on its output and
 blink the LED three times to indicate success.  If the supply cannot output
 enough power, the Sink will turn its LED on solid to indicate failure, and
 leave its output turned off.
+
+### Status LED descriptions
+
+LED State            | Mode  | Meaning
+---------------------|-------|---------------------------------------------------
+Fast blink (4/sec)   | Sink  | Negotiating USB Power Delivery contract
+Medium blink (2/sec) | Sink  | USB Power Delivery contract established, output on
+Off                  | Sink  | Output on
+On                   | Sink  | Unable to get configured power from source
+Slow blink (1/sec)   | Setup | Running in setup mode
+Fast blink (4/sec)   | Setup | `identify` command run