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.

Big Note is free and should do the very limited job for which it’s designed. But it is totally unsupported.

Any information entered in the Big Note field is stored only in the catalogue. It is never written to the images’ XMP files and is not included in files exported from LR. There are other limitations like the lack of any control over the type of data that can be entered in fields, and how Lightroom doesn’t permit plug-ins to use its built-in XMP-writing engine. So I have big reservations about how custom field metadata is handled in LR2, LR3, Lr4, Lr5….

The plug-in is unencrypted and can be an example 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. But I do not generally recommend the use of custom fields if you can’t support them yourself (see the LR SDK and particularly the reference manual).

Big Note is free and should do the very limited job for which it’s designed. But it is totally unsupported.