Added very important NOTE to README.md

[KingHowler]

I wanted to exclude the UNKOWN rank, but for some reason it didn't work.

After almost an hour of confused browsing through your code I found out that UNKNOWN is denoted by ?.

I have added this to your README.md in the appropriate section to ensure others don't have to go through the same.

[vercel]

@KingHowler is attempting to deploy a commit to the ryo-ma's Team Team on Vercel.

A member of the Team first needs to authorize it.

[KingHowler]

@ryo-ma, please check if this should be approved, this will save a lot newbies who don't know TS to use your project more easily