Big Note is a very simple free plug-in which adds a field to the Metadata panel where you can add notes about the picture. You can store any text in the field, and it can also be searched by smart collections.
Any information entered in the Big Note field is stored in the Lightroom catalogue. It is never written to the images’ XMP files and is not included in files exported from LR. That’s because Lightroom doesn’t yet permit a plug-in to use its XMP-writing engine, and it’s one reason why I have reservations about how custom field metadata is handled in LR2, LR3, Lr4, Lr5….
The plug-in is unencrypted and is a great start if you want to add your own custom fields to LR. You don’t have to know much about coding to modify these files. For example, it wouldn’t be difficult to figure out how to add a second custom field to your catalogue.
Big Note is free, and it is totally unsupported, but should do the very limited job for which it’s designed. Try it first on a test catalogue.