Here are some suggestions.

Give the option to backup / restore all data.
Currently the notes are tied to the specific urls it would be better to have an option to tie a note to a specific domain or broad url matching.
Also an option to see all the notes and highlights done by the app in option or configuration screen of the app.

Is there a github repo for this?

I reached out to Prateek about adding some features and he is very responsive, if not busy finishing his studies. The tool does what it says, my suggestion is for him to add an export selected text to the clipboard so it can easily be copied to another application. For examples apples notes or Evernote. Good luck Prateek.

I have been using it now for a while and it saves me a lot of time by highlighting all the important stuff that I might need to look again in future and notes feature is also too good.

Very useful and light weight

Easy to use : )

The best feature is that the highlight stays saved even after page reload or page revisit. One recommendation though, you can add a list of the urls I highlighted. Not easy to remember all the urls with notes and highlighted stuff you know. Rest of the functionality is pretty Good.