Make clearer what pycdio is needed for

Completely addresses thomasvs/morituri#96.

README.md

@@ -32,6 +32,7 @@ REQUIREMENTS
 - python-setuptools, for plugin support
 - python-cddb, for showing but not using disc info if not in musicbrainz
 - pycdio, for drive identification (optional)
+  - Required for drive offset and caching behaviour to be stored in the config file
 
 Additionally, if you're building from a git checkout:
 - autoconf

morituri/rip/drive.py

@@ -67,6 +67,7 @@ class Analyze(logcommand.LogCommand):
 
         info = drive.getDeviceInfo(self.options.device)
         if not info:
+            self.stdout.write('Drive caching behaviour not saved: could not get device info (requires pycdio).\n')
             return
 
         self.stdout.write(

morituri/rip/offset.py

@@ -239,6 +239,7 @@ CD in the AccurateRip database."""
 
         info = drive.getDeviceInfo(device)
         if not info:
+            self.stdout.write('Offset not saved: could not get device info (requires pycdio).\n')
             return
 
         self.stdout.write('Adding read offset to configuration file.\n')