Big Note
is a very simple free plugin which adds a custom field to the Metadata panel. This lets you 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.

I am cautious about encouraging people to enter custom metadata in Lightroom. It is stored only in the catalogue and is never written to the images’ XMP files or included in files exported from LR. That’s both good, in the sense of privacy, but bad in that the data is trapped inside the catalogue. So I do not generally recommend the use of custom fields if you can’t support them yourself.

The plugin code is unencrypted and might help you learn bits of Lua, LR’s scripting language (see the LR SDK and particularly the reference manual). You don’t have to know much about coding if, for example, you just want to add a second custom field to your catalogue.

But please don’t ask for help using it. Big Note is free and will do the limited job for which it’s designed.