Hello! I am the one behind this app. A friend who thinks I am not promoting it enough submitted it to HN (not complaining at all, thanks Aditya!).
I had also posted on the "What are you working on?" thread yesterday - https://news.ycombinator.com/item?id=43531684.
Happy to answer any questions.
Looks like a great app.
For those who can't read sheet music, can I suggest a guitar tabs?
I would suggest learning standard notation. Tabs are useful (since you often have a choice of what string to play any given note on, but which you choose matters both because different strings sound different, and because some strings will make the next note unreachable), but standard notation has benefits too. If you know both you have a chance of playing along with anyone else - most people will hand you standard notation when they want you to play background to solo, or play with in a group with you (this depends on what type of group - some groups will have tab some will not).