Add some "how" documentation

README.md

@@ -2,6 +2,25 @@ This is a [Factorio](http://www.factorio.com/) mod. It adds a small UI frame
 that informs you of the current state of any circuit signals attached to a
 "Production Monitor" entity.
 
+
+## How do I use this? ##
+
+The Production Monitor (prodmon) system consists of two parts:
+
+1. The titular production monitor, a combinator that can be hooked up by
+circuit wire to any network, and which can then see all the signals on the
+wire, and
+
+2. A HUD element showing every monitor's signals, with particular details:
+    - the signal's current value,
+    - percentage of fullness (learned from the signal's history),
+    - rate of change, and
+    - estimated time to full/empty (based on the rate of change).
+
+At present, there are no filters, so only show the monitor the signals you care
+about (use some other combinator to black- or whitelist as desired).
+
+
 ## Many thanks for ##
 
 * The [encouragement of the Reddit Factorio community](https://www.reddit.com/r/factorio/comments/6apj6l/who_uses_yarm_anymore/).