Added Features section to README, main.c docs

The README now outlines the features of the PD Buddy Firmware.

A bit of documentation was added to main.c, along with renaming the
pd_buddy() function to sink().

README.md

 
 This is the firmware for the PD Buddy project.  Currently, this specifically
 means the [PD Buddy Sink][].  The firmware is currently under heavy development,
-but is partially functional.
+but is largely functional.
 
 [PD Buddy Sink]: https://git.clayhobbs.com/pd-buddy/pd-buddy-sink
 
+## Features
+
+* Two boot modes, Setup and Sink, selected by the Setup button's state at
+  startup.
+* Sink mode Implements a simple USB Power Delivery sink, aiming for full
+  compliance with the USB Power Delivery Specification, Revision 2.0,
+  Version 1.3.
+* Requests the configured voltage and current if available, or a safe, low
+  power request otherwise.
+* Provides power on the output connector only when an explicit contract is in
+  place for the configured voltage and current.
+* Setup mode implements a USB CDC-ACM command-line interface allowing
+  configuration to be loaded from and stored in flash.
+
 ## Prerequisites
 
 To compile the firmware, you must first install the [GNU ARM Embedded

src/main.c

     usbStart(serusbcfg.usbp, &usbcfg);
     usbConnectBus(serusbcfg.usbp);
 
+    /* Start the shell */
     pdb_shell();
 }
 
 /*
  * Negotiate with the power supply for the configured power
  */
-static void pd_buddy(void)
+static void sink(void)
 {
     /* Start I2C2 to make communication with the PHY possible */
     i2cStart(&I2CD2, &i2c2config);
         setup();
     } else {
         /* Button unpressed -> deliver power, buddy! */
-        pd_buddy();
+        sink();
     }
 }