Added information about how PDOs will be printed

Still preliminary, but the general idea is that each PDO will be printed
similarly to how a configuration object is printed, but numbered and
indented to make a nice human- and machine-readable list.

docs/console_config.md

 # PD Buddy Sink Serial Console Configuration Interface
 
-Version 1.1.0-dev, 2017-07-27
+Version 1.1.0-dev, 2017-07-28
 
 The PD Buddy Sink can be put into setup mode by holding the Setup button while
 plugging it into a computer.  In this mode, the device runs a configuration
 ongoing run of Setup mode.  The output is disabled in Setup mode by default,
 and is always enabled in Sink mode.
 
+### get_source_cap
+
+Usage: `get_source_cap`
+
+Prints the capabilities advertised by the Power Delivery source.  Each
+capability is represented by a Power Data Object (PDO), printed in the format
+listed in the PDO Format section below.
+
 ## Configuration Format
 
 Wherever a configuration object is printed, the following format is used.
 The field's value is a floating-point decimal number, followed by a space and a
 capital A.  For example: `2.25 A`
 
+## PDO Format
+
+Wherever a PDO is printed, the following format is used.  
+
+A PDO consists of a number of fields, one per line.  Each field is of the
+format:
+
+    name: value
+
+When a list of PDOs is printed, each PDO is numbered with a line as follows:
+
+    PDO #1:
+
+The fields of such a PDO are each indented by a single ASCII tab character.
+
+**TODO**: describe exactly how each type of PDO is printed.
+
 ## USB Descriptors
 
 The PD Buddy Sink can be identified by the following USB device descriptors: