Save a personal comment for an app

**Is your feature request related to a problem? Please describe.**
I forget things easily. I want to save a brief text for the future myself to
- not periodically retry apps that I don't like
- (when periodically retrying an app intentionally) remind of a specific issue that I want to keep an eye on
- remember why I ignored an app update (the situation may have changed after a couple years, or F-droid may not have kept up with the upstream releases)
- (when updating an app) mention an issue I reported, to report upstream whether it is solved

**Describe the solution you'd like**
I want to save a free text associated with a specific app.

**Describe alternatives you've considered**
I have a separate text file, but it is not convenient.