Added documentation for configuration format

It's very important to document the format in which configuration is
printed, since being able to reliably read the configuration is
essential to implementing configuration GUIs.  Now it's documented, and
the format is considered stable.

docs/console_config.md

 
 Blinks the LED quickly.  Useful for identifying which device you're connected
 to if several are plugged in to your computer at once.
+
+## Configuration Format
+
+Wherever a configuration object is printed, the following format is used.
+
+The configuration consists of a number of fields, one per line.  Each field is
+of the format:
+
+    name: value
+
+Only the `status` field is mandatory.  Any or all other fields may be absent if
+their values are not valid or relevant.
+
+### status
+
+The `status` field holds the name of the status of the printed configuration
+object.  The possible names are:
+
+* `empty`: A configuration object left empty after the last erase.
+* `valid`: The configuration object that holds the current device settings.
+* `invalid`: A configuration object that once held settings, but has been
+  superseded.
+
+### flags
+
+The `flags` field holds zero or more flags.  If no flags are enabled, the
+field's value is `(none)`.  Otherwise, the field's value is some combination of
+the following words, separated by spaces, representing the flags enabled in
+this configuration object:
+
+* `GiveBack`: allows the power supply to temporarily reduce power to the device
+  if necessary.
+
+### v
+
+The `v` field holds the fixed voltage of the configuration object, in volts.
+The field's value is a floating-point decimal number, followed by a space and a
+capital V.  For example: `20.00 V`
+
+### i
+
+The `i` field holds the fixed current of the configuration object, in amperes.
+The field's value is a floating-point decimal number, followed by a space and a
+capital A.  For example: `2.25 A`